#include <algorithm>
#include <iostream>
#include <iomanip>
#include <cstring>
#include <string>
#include <vector>
#include <queue>
#include <cmath>
#include <stack>
#include <set>
#include <map>
typedef long long ll;
typedef unsigned int uint;
using namespace std;

int main() {
	int k, n, f;
	cin >> k >> n >> f;
	int a[f];
	int sum = k * n;
	for (int i = 0; i < f; i++) {
		cin >> a[i];
	}


	int out = 0;
	for (int i = 0; i < f; i++) {
		if (out + a[i] > sum){
			cout << -1 << endl;
			return 0;
		}
		out += a[i];
	}
	cout << sum - out << endl;
}