#include <bits/stdc++.h>
using namespace std;

int main() {
    int n, m, a, sum = 0;
    cin >> n >> m;
    for (int i = 0; i < m; i++) {
        cin >> a;
        sum += a;
    }
    if (sum <= n) cout << n - sum << endl;
    else cout << -1 << endl;
}