#define _USE_MATH_DEFINES
#include<stdio.h>
#include<string>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include<iomanip>
//#include<bits/stdc++.h>


using namespace std;



int main(){
	int a,b,c;
	cin>>a>>b>>c;
	a*=b;
	for(int i=0;i<c;i++){
		int x;
		cin>>x;
		a-=x;
	}
	if(a<0)a=-1;
	cout<<a<<endl;
	return 0;
}