#include <bits/stdc++.h>
int main() {
  int day;
  int homework;
  int homework_day[10001];
  int i = 0;
  int homework_day_ruiseki = 0;
  int play = 0;
  scanf("%d %d", &day, &homework);
  while (i < homework) {
    scanf("%d", &homework_day[i]);
    i++;
  }
  i = 0;
  while (i < homework) {
    homework_day_ruiseki += homework_day[i];
    i++;
  }
  play = day - homework_day_ruiseki;
  if (play <= -1) play = -1;
  printf("%d\n", play);
  return 0;
}
