int main()
{
    int n;
    string s1, s2;
    cin >> n >> s1 >> s2;
    s1 += s1;
    int pos = (int) s1.find(s2);
    switch ( pos )
    {
    case -1:
        cout << -1;
        break;
    case 0:
        cout << 0;
        break;
    default:
        cout << n-pos;
    }
    return 0;
}

Unfortunately, your solution has been rejudged. :) string.find() is too slow. Good luck!

LOL