A — Vasya and the Bus

Firstly, if n = 0, then children can't be in the bus, so if m = 0 then the answer is (0, 0), otherwise the answer is "Impossible". Now n > 0. If m =  = 0, than it is only one possible variant of passage — the answer is (n, n). Otherwise, more grown-up take some children, less the sum that people pay. So, if only one adult takes all children, than we get maximal sum — n + m - 1. Maximum min(n, m) adults can take the children with them, so the minimal answer is n + m - min(n, m) = max(n, m).

B — Surrounded

Let's find the minimum distance between two circles L. Then the answer to our problem is L / 2.
Now d is the distance between the centers of the circles, R, r — their radiuses. There are 3 possible cases:
- Circles don't intersect. Then L = d - R - r. Firstly, it's reachable: let's consider the segment, connecting the centers of the circles, and take its part, which is out of both circles — its length is exactly d - R - r.

Let's prove that lesser distance is impossible. If the segment connecting two points of distinct circles have length l, than R + l + r >  = d, so l >  = d - R - r.

- If one circle is into another, than analogically the answer is R - d - r, where R is the radius of the bigger circle.
- If the circles intersect, then the answer is 0.

C — STL

In this problem we have an array of strings (s₁, s₂, ...s_n), where s_i  = pair or int.
Let's consider bal_i = the difference between number of "pair" and "int" int the subarray (s₁, s₂, ...s_i).
Than we can prove that the type can be reestablished from the array s  <  =  >  bal_i >  = 0 for 1 <  = i < n and bal_n =  - 1. This can be proved using mathematical induction, the parameter is the number of "int"-s in the array.
And how to find the solution, if we know that is exists? Consider the function gettype(inti), which builds the type beginning in the position i and returns the index, next to the position where it finished building of type. How it works? If s_i =  "int", then the function prints "int" and returns i + 1. Else it prints "pair < ", gettype(i + 1) is launched, let it returns j. Then we print ", " and launch gettype(j) (it returns k), then we print " > " and return k.
We can not to do check if the type can be reestablished from the array in the beginning: launch gettype() and print "Error occurred" if there became some contradiction during the building of type.

D — Non-secret Cypher

First solution: Let's use the method of two pointers. For every number we will know how many times it occurs in the current segment [l, r]. For fixed l we increase r until a[r] occurs in the current segment less than k times. If a[r] occurs int the segment [l, r] k times, we add to the answer all segments [l, t] for all t >  = r and increase l (and do not forget to decrease the number of a[l] in the current segment).
To keep the number of every value in the segment we can use map or compression of the coordinates. Also it's important not to forget that the maximal answer is , which doesn't fit in int.
Second solution: firstly, let's do the compression of the coordinates. For every value X we write the list of the positions i such that a[i] = X. Now using that we fill the array b: b[i] is the minimum index that segment [i, b[i]] contains k numbers equal to a[i] (obviosly, a[b[i]] = a[i]), if this index doesn't exist, then b[i] = n + 1.
Let's find for every index i a minimal index j such that segment [i, j] contsins k equal numbers (if such j doesn't exist, we say that j = n + 1) — then we add to the answer n + 1 - j. This j is equal to . All that minimums can be found in a single pass through a from its end. Then we can sum the answers for all indexes 1 <  = i <  = n.
The complexity of these solutions is O(nlogn).

E — Counter Attack

This problem has several different solutions. Anyway, we should consider towns as the vertices of the graph, roads as its edges. Now we are to find the connected components of the complementary graph (CG) of the given graph. Let's take the vertex A with the minimal degree c: . We call the set of the vertices who are connected with A — B, that are not connected — D . All vertices which don't connect with A will be in the same connected component of the CG. Let's build the complemented graph to the subgraph including A and set B — there are vertices and  =   =  O(m) edges.
Let's build DSU for the vertices of given graph and merge components which we have found using the new-built graph. All that we should do after it is to consider vertices from B: now we look to the vertex v. We put v in the same component as A if v has edges not to all vertices of D (we do it using DSU too). The complexity of this solution is O(m) or O(mlogn).

Another solution: we keep set a, which contains vertices, which haven't been visited yet. Let's run series of bfs to find the components of CG. If we are working now with the vertex v, pass through a. Consider u is the element of a. If edge (u, v) isn't in the given graph, than u is in the same component of CG as v, so, we can erase it from a and add to queue. Otherwise, we do nothing.
What is the complexity of this solution? To understand that, we must know how many times whe consider fixed vertex u in the set a. u remains int a when we run to them from vertex v  <  =  >  edge (u, v) is in the given graph. So, every vertex u remains in a no more than its degree. So, the complexity of this algorithm is O(mlogn) (log n because of binsearch — we need use it to know, is (u, v) in the graph). If we use HashMap, we'll get O(m).