#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint1000000007;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000000000000000

int main(){
	
	int K,N,F;
	cin>>K>>N>>F;
	
	K *= N;
	
	rep(i,F){
		int a;
		cin>>a;
		K -= a;
	}
	if(K<0)K = -1;
	
	cout<<K<<endl;
	
	return 0;
}