#include <bits/stdc++.h>

using namespace std;

int main()
{
	int N, K, F;
	cin >> K >> N >> F;
	vector<int> A(F);
	int require = 0;
	for (int i = 0; i < F; i++)
	{
		cin >> A[i];
		require += A[i];
	}

	int ans = N * K - require;
	if (ans < 0)
	{
		cout << -1 << endl;
	}
	else
	{
		cout << ans << endl;
	}
}