#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <cmath>
#include <climits>
#include <iomanip>
#include <set>
#include <map>
using namespace std;
typedef long long ll;
int main(){
    ll n,a;
    cin >> n >> a;
    a *= 2;
    ll k = n / a;
   if(n % a == 0)k--;
    cout << k * a / 2 << endl;
}