#include <iostream>
using namespace std;

int main()
{
	int k, n, f, a, diff = 0;
	
	cin >> k >> n >> f;
	
	for (int i = 0; i < f; i++){
		cin >> a;
		diff += a;
	}
	
	cout << k * n - diff << endl;
	
	return (0);
}