#include <bits/stdc++.h>
int main() {
  int N, M;
  scanf("%d %d", &N, &M);
  for (int i = 0; i < M && N >= 0; ++i) {
    int tmp;
    scanf("%d", &tmp);
    N -= tmp;
  }
  printf("%d\n", N >= 0 ? N : -1);
  return 0;
}
