#include <stdio.h>

int main(){

int i;
int K,N,F,Age;
int sum=0;

scanf("%d",&K);
scanf("%d",&N);
scanf("%d",&F);
for(i=0;i<F;i++){
  scanf("%d",&Age);
  sum+=Age;
}
if(K*N - sum < 0)
  printf("-1");
printf("%d",K*N - sum);
}