#include<iostream>
#include<queue>
#include<cstdio>
#include<utility>
#include<algorithm>
#include<cmath>
using namespace std;
int main(){
	int k,n,f;scanf("%d%d%d",&k,&n,&f);
	int sum=k*n;
	int are;
	for(int i=0;i<f;i++){
		scanf("%d",&are);
		sum-=are;
	}if(sum<0)sum=-1;
	cout<<sum<<endl;
}