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

int main() {
	int K, N, F;
	cin >> K >> N >> F;
	int ans = K * N;
	for (int i = 0; i < F; i++)
	{
		int A;
		cin >> A;
		ans -= A;
	}
	if (ans < 0) ans = -1;
	cout << ans << endl;
}