Forest density

#include <iostream>
using namespace std;
int main(){
    long long n,m;
    cin >> n >> m;
    int prefix_sum[n+1][m+1];
    for (int i = 1; i <= n; i++) {
        string row;
        cin >> row;
        for (int j = 1; j <= n; j++) {
            int t = 0;
            if (row[j - 1] == '*') {
                t = 1;
            }
            prefix_sum[i][j] = t + prefix_sum[i - 1][j] + prefix_sum[i][j - 1] - prefix_sum[i - 1][j - 1];
        }
    }
    int ans[m];
    for (int i = 0; i < m; i++) {
        int y1, x1, y2, x2;
        cin >> y1 >> x1 >> y2 >> x2;
        ans[i] = prefix_sum[y2][x2] - prefix_sum[y1 - 1][x2] - prefix_sum[y2][x1 - 1] + prefix_sum[y1 - 1][x1 - 1];
    }

    for (int i = 0; i < m; i++) {
        cout << ans[i] << endl;
    }

    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: RUNTIME ERROR

Test 3

Verdict: RUNTIME ERROR