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

int main(void)
{
    int plength,elength,yuulength=0;;
    scanf("%d %d",&plength,&elength);
    while(1)
    {
        plength-=elength*2;
        if(plength<=0)
        {
            break;
        }
        yuulength+=elength;
    }
    printf("%d",yuulength);
}