#include <stdio.h>

int main(){
	int grain,bag,family;
	scanf("%d%d%d",&grain,&bag,&family);
	
	int sum = 0;
	for(int i = 0;i < family;i ++){
		int tmp = 0;
		scanf("%d",&tmp);
		sum += tmp;
	}
	
	if(grain * bag >= sum){
		printf("%d",(grain * bag) - sum);
	}else{
		printf("-1");
	}
}