#include<iostream>
#include<string>
#include<math.h>
using namespace std;

int main() 
{
	int a, b, c,sum;
	cin >> a >> b >> c;
	sum = a * b;
	for (int i = 0; i < c; i++) {
		int s;
		cin >> s;
		sum = sum - s;
	}
	if (sum >= 0) {
		cout << sum;
	}
	else {
		cout << -1;
	}
}