#include <iostream>
#include <algorithm>
using namespace std;

int main() {
	int a, b, c;
	cin >> a >> b >> c;
	a *= b;

	for(int i=0; i<c; i++) {
		int d;
		cin >> d;
		a -= d;
	}

	if(a < 0) {
		a = -1;
	}

	cout << a << endl;
}