#include <bits/stdc++.h>
using namespace std;

int main() {
	int K, N, F;
	cin >> K >> N >> F;

	int fs = 0;
	for (int i = 0; i < F; i++) {
		int n;
		cin >> n;
		fs += n;
	}

	int m = K * N;
	if (m >= fs) {
		cout << m - fs << endl;
	}
	else {
		cout << -1 << endl;
	}
}