#include<stdio.h>

int main(void){
	int k,n,f,i,sum=0,ans=0;

	scanf("%d %d %d",&k,&n,&f);

	int age[f];

	for(i=0;i<f;i++){
		scanf("%d",&age[i]);
		sum += age[i];
	}

	if(sum > k*n) ans = -1;
	else ans = k*n - sum;

	printf("%d\n",ans);

	return 0;
}