#include<stdio.h>

int main(){
	int k,n,f,a,s=0;
	long all;
	scanf("%d %d %d",&k,&n,&f);
	all=(long)k*(long)n;
	for(k=0;k<f;k++){
		scanf("%d",&a);
		s+=a;
	}
	printf("%ld\n",(all<s)?-1:all-s);
	return 0;
}