#include<stdio.h>

#define MOD(m) ((m)%1000007)

int main(void){

	int k,n,f;
	scanf("%d%d%d",&k,&n,&f);
	int sum=0;
	while(f>0){
		int t;
		scanf("%d",&t);
		sum+=t;
		f--;
	}
	if(k*n<sum){
		printf("-1\n");
	} else {
		printf("%d\n",k*n-sum);
	}
	return 0;
}