#include <iostream>

using namespace std;

int main(int argc, const char* argv[])
{
	int N, K, F;
	cin >> N >> K >> F;
	int A[F];
	for (auto&& a : A) cin >> a;
	int n = N * K;
	for (auto&& a : A) n -= a;
	cout << (n >= 0 ? n : -1) << endl;
	return 0;
}