#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES
#include <iostream>
#include <map>
#include <set>
#include <list>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <string>
#include <vector>
#include <algorithm>
#include <utility>
#include <queue>
#include <iomanip>

#define D(d) fixed<<setprecision(d)

using namespace std;


int main(void)
{
	int k, n, f;

	cin >> k >> n >> f;

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

	if (k * n > sum) cout << k * n - sum  << endl;
	else cout << -1 << endl;

	

	return (0);
}