#include <stdio.h>

int main(){

int i;
int K,N,F,Age=0;
int sum=0;
int a =-1;

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("%d",a);
else
printf("%d",K*N - sum);
}