#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<math.h>


int main(void){
int l,k;
int a=0;
scanf("%d %d",&l,&k);

for(;;){
    if(l-k-k<=0){printf("%d\n",a);return 0;}
    else{a=a+k;l=l-k-k;}
}
}