#include<stdio.h>
int main(void){
	int k, n, f, a[100], i, sum, age = 0;
	scanf("%d%d%d", &k, &n, &f);
	sum = k * n;
	for(i = 0 ; i < f ; i++){
		scanf("%d", &a[i]);
		age += a[i];
	}
	if(sum < age){
		printf("%d", -1);
	}else{
		printf("%d", sum - age);
	}
	return 0;
}