#include <stdio.h>

int main(void)
{
  int k, n, f, a , age, answer;
  scanf("%d%d%d", &k, &n, &f);
  for(; f > 0; f--){
    scanf("%d", &a);
    age += a;
  }
  answer = (k * n) - age;
  if(answer < 0) answer = -1;
  printf("%d\n", answer);

  return 0;
}