#include <algorithm>
#include <iostream>
#include <fstream>
#include <cstdio>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <cmath>
#include <functional>
#include <cassert>
#include <iomanip>
#include <array>
#include <time.h>
#include <limits.h>
#include <numeric>
#define debug(x) cout<<#x<<": "<<x<<endl
#define rep(i,n) for (int i=0;i<(n);i++)
#define FOR(i,a,b) for (int i=(a);i<=(b);i++)
using namespace std;

typedef vector<int> vi;


void solve(){
#ifdef _WIN32
	istream &cin = ifstream("input.txt");
#endif
	int k, n, f, sum = 0;
	cin >> k >> n >> f;

	rep(i, f){
		int tmp;
		cin >> tmp;
		sum += tmp;
	}

	cout << (k * n < sum ? -1 : k * n - sum) << endl;

}

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);

	solve();
	system("PAUSE");
	return 0;
}