If the length of the input strings are not equal, we deduce at once "-1 -1" and complete the program. Otherwise, let the number n is equal to the length of input lines.

We iterate through the number i. Let us learn in O (1) to understand for some of the smallest substring j b [0 ... n - i - 1] can be represented as a [i 1 ... j - 1] r (a [j. .. n - 1]). To do this, calculate the prefix function (p [i]) for the string s1 = r (a) '\ 0' b and the z-function (z [i]) for the string s2 = b '\ 0' a. It is clear that for fixed i, j is the required value will be n - p [2n - i - 1], with the substring a [i 1 ... j - 1] and b [0 .. j - i] must be the same (1). The last statement is easily verified, using the calculated z-function. Is also trivial proof of the fact that if a fixed i the property (1) does not hold for the chosen j, then it will fail for large j.

The asymptotic complexity of the solution - O (n).