【題解】AtCoder ABC 163B – Homework

clude <iostream>
#include <math.h>
using namespace std;
 
int n, m, a;
 
int main() {
    cin >> n >> m;
    for (int i = 0; i < m; i++){
        cin >> a;
        n -= a;
    }
    if (n < 0) cout << -1 << "\n";
    else cout << n << "\n";
}