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