#define _USE_MATH_DEFINES	//M_PI
#include <iostream>			//std::cout, std::cin
#include <string>			//std::string
#include <vector>			//std::vector
#include <valarray>			//std::valarray	数値のみの一次配列
#include <algorithm>		//std::sort
#include <time.h>			//localtime_s
#include <cstdlib>			//abs
#include <cmath>			//abs, std::pow, sqrt, sin, cos,
#include <fstream>			//std::ifstream
#include <iomanip>			//std::setprecision
#include <random>			//std::random(C++11)
#include <numeric>			//std::accumulate

int main(void) {

	//test用
	//std::ifstream in("test.txt");
	//std::cin.rdbuf(in.rdbuf());

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

	int temp;
	int sum = 0;
	for (int i = 0; i < F; i++) {
		std::cin >> temp;
		sum += temp;
	}

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

}