#include<stdio.h>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <algorithm>
#include <sstream>
#include <iostream>
#include <string>
#include <stdio.h>

using namespace std;

int main() {

	int K, N, F;
	cin >> K >> N >> F;
	int sum = K * N;
	for (int i = 0; i < F; i++) {
		int t;
		cin >> t;
		sum -= t;
	}

	if (sum < 0) cout <<-1 <<endl;
	else cout << sum << endl;

	return 0;
}