#include <iostream>
#include <vector>
#include <numeric>

typedef unsigned int uint;

int main() {
	int k, n, f;
	std::cin >> k >> n >> f;

	std::vector< int > a;

	for (uint i=0; i<f; ++i) {
		int tmp = 0;
		std::cin >> tmp;
		a.push_back(tmp);
	}

	int remain = (k * n) - std::accumulate(a.begin(), a.end(), 0);
	std::cout << (remain >= 0 ? remain : -1) << std::endl;
}