First of all, you should better share your codes using numbers of submissions like that 120678373. It is more convenient for CF users; I examined your codes this way due to "compare" button. Also, it seems that you mixed up the submissions: the one you suggest as WA (https://ide.codingblocks.com/s/579801) is actually AC (my AC submission 120693832).

The actual reason of WA is that int * int == int, so a[i].first * a[j].first behaves not as you think it does. Why do you have AC and WA submissions both?

The compilation process in C++ consists of many stages. One of the earliest just scans the code and once it finds a #define, it will replace all the occurrences of the defined word below. typedef creates a synonym for the type; it behaves in more clever way.

So this code would get compilation error

#define node pair<int, int>
...
int node = 6;

because preprocessor would replace node with pair<int, int> and there are many symbols that aren't allowed in variable names. On the other side, this code would work fine

typedef pair<int, int> node;
...
int node = 6;

The compiler understands that node is a variable name and doesn't replace it with something else.

So, in your AC submission the actual type of array a is array of pair<long long, long long>. You have these two defines in your code:

#define int long long
#define node pair<int, int>

and they do combine to the following (the first one changes the second one)

#define node pair<long long, long long>

And your WA submission contains pair<int, int> a[n], so that may cause WA or even TLE (in the case if (a[i].first * a[j].first > 2 * n) works too rare).