#include <iostream>
using namespace std;

int main(){
	int L, K;
	cin >> L >> K;
	
	if(L % (2 * K) == 0){
		cout << ((L / (2 * K)) - 1) * K << endl;
	} else {
		cout << (L / (2 * K)) * K << endl;
	}
	
	return 0;
}