#include <bits/stdc++.h>
int main() {
  long int ans, N, M, A[10000], sum = 0;
  scanf("%ld", &N);
  scanf("%ld", &M);
  for (int a = 1; a <= M; a++) {
    scanf("%ld", A[a - 1]);
    sum += A[a - 1];
  };
  ans = N - sum;
  if (ans < 0) ans = -1;
  printf("&ld", ans);
};
