#include <iostream>
#include <vector>

using namespace std;
int main()
{
	vector<int> f;
	int a,b,c;
	int s=0;
	cin >> a >> b >> c;
	f.resize(c);
	
	for (int i = 0; i < c; i++)
	{
		cin >> f[i];
	}

	for (int i = 0; i < c; i++)
	{
		s += f[i];
	}

	if (a* b >= s)
	{
		cout << (a * b) - s << endl;
	}

	else cout << -1 << endl;

	return 0;
}