#include <bits/stdc++.h>
int main() {
  int day, work, i, sum = 0, fun;
  int *d;
  d = (int *)malloc(sizeof(int) * work);
  scanf("%d%d", &day, &work);
  for (i = 0; i < work; i++) {
    scanf("%d", d[i]);
    sum += d[i];
  }
  fun = day - sum;
  if (fun < 0) {
    printf("-1\n");
  } else {
    printf("%d", &fun);
  }
  return 0;
}
