Is it a Tree?: e-olymp

#include <iostream>
using namespace std;

int main() {
    int x;
    cin >> x;
    int tot = 0;
    for (int i=0; i < x*x; i++) {
        int y;
        cin >> y;
        if (y == 1) {
            tot += 1;
        }
    }

    if (tot == 2*(x-1) {
        cout << "YES" << endl;
    }

    else {
        cout << "NO" << endl;
    }
    return 0;
}