#include <iostream>
#include <string>
#include <algorithm>
#define MAX 300005
using namespace std;
string T, P;
int b[MAX], n, m;
int i, j;

void kmpPre() {
    i = 0, j = -1;
    b[0] = -1;
    while (i < m) {
        while (j >= 0 && P[i] != P[j]) j = b[j];
        i++; j++;
        b[i] = j;
    }
}

void kmpSearch() {
    i = 0, j = 0;
    while (i < n) {
        while (j >= 0 && T[i] != P[j]) {
            j = b[j];
        }
        if(i == n -1) {
            return;
        }
        i++;
        j++;
    }
}

int main() {
    int N;
    cin >> N >> P;
    T = P;
    reverse(T.begin(), T.end());
    n = T.length();
    m = P.length();

    swap(T, P);
    kmpPre();

    kmpSearch();

    cout << m - j - 1 << endl;

    return 0;
}