#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

int main(){
  int n,d;
  cin>>n>>d;
  if(n<=d){
    cout<<0<<endl;
    return 0;
  }
  int x=(n-1)/d;
  int y=(n-1)%d;
  int z=1;
  if(d-y>y){
    z=d-y;
  }
  z*=x;
  cout<<z<<endl;
  return 0;
}