String Tale (KMP)

#include <bits/stdc++.h>

typedef long long ll;
using namespace std;

#define f(i,a,b) for(int i = (a); i <= (b); i++)

const ll MOD = 1000000007;

int F[250005], N;
string S, T;

int main()
{
    cin >> N >> S >> T;
    S += S;
    F[0] = -1, F[1] = 0;
    f(i,2,N)
    {
        int match = F[i-1];
        while(match >= 0 && T[match] != T[i-1]) match = F[match];
        F[i] = match+1;
    }
    int match = 0;
    f(i,0,2*N-1)
    {
        while(match >= 0 && S[i] != T[match]) match = F[match];
        match++;
        if(match == N)
        {
            if(i == N-1) cout << 0;
            else cout << 2*N-i-1;
            return 0;
        }
    }
    cout << -1;
}