#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main(int argc, char**argv)
{
	int mame, hukuro, ninzu;
	cin >> mame >> hukuro >> ninzu;

	int sum_age = 0;
	vector<int> age_vec;
	for (int ii=0; ii<ninzu; ++ii) {
		int age; cin >> age;
		age_vec.push_back(age);
		sum_age += age;
	}
	int sum_mame = (mame*hukuro);
	if (sum_age > sum_mame) {
		cout << -1 << endl;
	}
	else {
		cout << (sum_mame-sum_age) << endl;
	}
	return 0;
}