# include <iostream>
# include <fstream>
# include <cmath>
# include <vector>
# include <algorithm>
# include <math.h>
# include <time.h>
# include <string>
# include <string.h>
# include <map>
# include <queue>
# include <stack>

using namespace std;
const int MAXN = 1000000007;//, Maxn = 1000000009;

long long n, a, san, num, dereje = 1, x;

bool zero, one;

string s, h;

int main()
{
    cin >> n >> s >> h;

    a = n-1;

    for (int i=0; i<n; i++) {
        san = (san + (dereje * (s[i]-'a'+1))) % MAXN;
        num = (num + (dereje * (h[i]-'a'+1))) % MAXN;
        dereje = (dereje * 27) % MAXN;
    }

    if (san == num)    {cout << 0 << '\n'; return 0;}

    for (int i=0; i<n; i++) {
        san = (san * 27) % MAXN,
        san = (san - (((s[a]-'a'+1) * dereje) % MAXN)),
        san = (san + MAXN) % MAXN,
        san = (san + (s[a]-'a'+1)) % MAXN;

        if (san == num)    {cout << i+1 << '\n'; return 0;}
        a--;
    }

    cout << -1 << '\n';

    return 0;
}


WA 8.why?
i used HASH!


plz help me!