#define _USE_MATH_DEFINES
#include<stdio.h>
#include<string>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include<iomanip>
using namespace std;
int x[100008];
int y[100008];

int main(){
	int n,g;
	cin>>n>>g;
	g*=2;
	int y=n/g;
	if(!y){
		cout<<y<<endl;
		return 0;
	}
	if(!(n%g))y--;
	cout<<y*(g/2)<<endl;
	



	return 0;
}