#include <bits/stdc++.h>
int main() {
  int N, M;
  int A[1001];
  int i;
  int count = 0;
  scanf("%d %d", &N, &M);
  for (i = 0; i < M; i++) {
    scanf("%d", &A[i]);
    count += A[i];
  }
  if (count > N) {
    printf("-1");
  } else {
    printf("%d", N - count);
  }
  return 0;
}
