#include <bits/stdc++.h>
#include "bits/stdc++.h"
#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <string>
#include <sstream>
#include <cmath>
#include <stack>
#include <queue>
#include <cctype>
#include <stdio.h>
#include <map>
#include <unordered_map>
#include <string.h>
#include <utility>

typedef long long ll;
const int INF = 1e8;

#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
using namespace std;

typedef pair<int, int> P;



int main() {

	int k,n,f;
	cin>>k>>n>>f;

	int sum=0;
	int tmp=0;
	for(int i=0; i<f; i++){
		cin>>tmp;
		sum +=tmp;
	}

	if(k*n-sum >=0){
		cout<<k*n-sum<<endl;
	}else {
		cout<<-1<<endl;
	}

	return 0;
}