#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <vector>

int PockyGame(int x, int y)
{

  if ( x <= 2*y ){
    return 0;
  }
  
  for(int i=1;;i++){    
    if ( x <= y*(i+1) ){
      return y*i/2;
    }
  }
}

int main(void)
{
  int a,b,c;
  scanf("%d",&a);
  scanf("%d",&b);
  c = PockyGame(a,b);
  printf("%d\n",c);
  return 0;
}