#include<stdio.h>
#include<algorithm>
using namespace std;
int main(){
	int a,b,c;scanf("%d%d%d",&a,&b,&c);a*=b;
	for(int i=0;i<c;i++){
		int d;scanf("%d",&d);a-=d;
	}
	if(a<0)a=-1;
	printf("%d\n",a);
}