#include "iostream"
using namespace std;

int N, F, K;
int age[2000];
int ans;

int main() {
	cin >> N >> F >> K;
	ans = N*F;
	for (int i = 0; i < K; i++) {
		cin >> age[i];
		ans -= age[i];
	}
	if (ans < 0)cout << "-1";
	else cout << ans;
	cout << "\n";
	return 0;
}