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

int main() {
unsigned long int n,k,c,a,b=0;
scanf("%lu%lu",&n,&k);
a=(k*2)+1;

do
{
n=n-a;
b=b+1;
}
while(n>=a);

c=n+b;
printf("%lu",c);
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */    
    return 0;
}