#include <stdio.h>

int main(void){
  int k,n,f,a,sum;
  scanf("%d %d %d",&k,&n,&f);
  for(int i = 0; i < f; i++){
    scanf("%d",&a);
    sum += a;
  }

  if(k * n - sum < 0)printf("-1\n");
  else printf("%d\n",k * n - sum);

  return 0;
}