#include<stdio.h>

int main(void){
	int k,n,f,i,bs = 0,bn;
	scanf("%d%d%d",&k,&n,&f);
	for(i = 1;i <= f;i++){
		scanf("%d",&bn);
		bs+=bn;
	}
	if(k*n >= bs){printf("%d\n",(k*n)-bs);}
	else{printf("-1\n");}
	return 0;
}