//#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 L, K;
	std::cin >> L >> K;

	if (L % (2 * K) != 0) {
		std::cout << (L / (2 * K))*K << std::endl;
	}
	else {
		std::cout << (L / (2 * K) - 1)*K << std::endl;
	}
}