#include <iostream>
#include <numeric>
#include <algorithm>
#include <string>
#include <vector>

#define rep(i, a) FOR(i, 0, a)
#define FOR(i, a, b) for(int i = a; i < b; ++i)

int k, f, n, sum = 0;
int a[101];

int main(){
	std::cin >> k >> f >> n;
	rep(i, n)std::cin >> a[i];
	sum = std::accumulate(a, a + n, 0);
	if (sum > k * f)std::cout << -1 << std::endl;
	else std::cout << k * f - sum << std::endl;

	return 0;
}