【題目敘述】https://atcoder.jp/contests/abc163/tasks/abc163_b
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";
}