#include <bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,j)  for(int i=0; i<j; i++)
#define vi vector<int>
#define vvi vector<vector<int>>
#define sr(a) sort(a.begin(),a.end())

signed main(){
  int l,k; cin>>l>>k;
  if(l%k==0){
    int g=l/k;
    if(g%2==0){
      cout<<(g/2-1)*k;
    }
    else{
      cout<<g/2*k<<endl;
    }
  }
  else{
    int g=l/k;
    cout<<g/2*k<<endl;
  }
}