#include <stdio.h>

int main(){
  int length, pace, times;
//  printf("input length ; ");
  scanf("%d", &length);

//  printf("input pace ; ");
  scanf("%d", &pace);

  times = (length-1)/(2*pace);
  if(length % (2 * pace) == 0)
    times --;
  printf("%d\n", (length - 1)/pace);
  return 0;
}