#include<stdio.h>

int main(void)
{
  int K,N,F;
  int A[100];
  int i;
  int bean;
  int ege = 0;

  scanf("%d %d %d", &K, &N, &F);
  
  for(i = 0; i < F; i++){
    scanf("%d", &A[i]);
    ege += A[i];
  }
  
  bean = K * N;

  if(bean < ege){
    bean = -1;
  }
  else{
    bean = bean - ege;
  }
  
  printf("%d\n", bean);
  
  

  return 0;
}