#include <bits/stdc++.h>
int main(void) {
  int n, m;
  int sum = 0;
  scanf("%d %d", &n, &m);
  int c[m];
  for (int i = 0; i < m; i++) {
    scanf("%d", &c[i]);
    sum += c[i];
  }
  n -= sum;
  if (n < 0)
    printf("-1");
  else
    printf("%d", n);
  return 0;
}
