#include <iostream>	//cin, cout
#include <vector>	//vector
#include <algorithm> //sort,min,max
#include <string>	//string
#include <ios>		//fixed
#include <iomanip>	//setprecision
#include <utility> //swap
#include <cstdlib>	//abs(int)
#include <cmath>	//sqrt
#include <sstream>	//stringstream,getline

using namespace std;

inline std::vector<std::string> Split_String(const std::string& str, const char key) {

	std::vector<std::string> result;
	std::stringstream SS(str);
	std::string temp;

	while (std::getline(SS, temp, key)) {
		result.push_back(temp);
	}

	return(result);

}

int main() {

	int K, N, F;
	cin >> K >> N >> F;

	int A = 0, temp = 0;
	for (int i = 0; i < F; i++) {
		cin >> temp;
		A += temp;
	}

	if (K * N >= A) {
		cout << K * N - A << endl;
	}
	else {
		cout << -1 << endl;
	}

	return 0;

}