#include <iostream>
using namespace std;

#define N 1000

int x, y, n;
int age[N];
char op[N-1];
int na[N][100001];

int main()
{
	int z = 0;
	cin >> x >> y >> n;
	for ( int i = 0; i < n; i++ ) {
		int t;
		cin >> t;
		z += t;
	}

	z = x * y - z;

	cout << (z >= 0 ? z : -1) << endl;

	return 0;
}