#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>

using namespace std;

int main(){
    int l,k;
    
    scanf("%d%d",&l,&k);
    
    printf("%d\n",(l-1)/(2*k)*k);
    
    return 0;
}