#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 ai;
    cin >> ai;
    sum += ai;
  }
  if (n < sum)
    cout << "-1" << endl;
  else
    cout << n - sum << endl;
  return 0;
}
