#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  cin >> n >> m;
  int sum = 0;
  for (int i = 0; i < m; ++i) {
    int a;
    cin >> a;
    sum += a;
  }
  if (sum <= n)
    cout << n - sum;
  else
    cout << -1;
  return 0;
}
