#include <iostream>
#include <vector>
#include <algorithm>
#include <math.h>

#define ALL(x) (x).begin(),(x).end()

using namespace std;
int main(){
    int l,k,ans = 0;
    cin >> l >> k;
    while(1){
        l -= 2*k;
        if(l <= 0)break;
        ans += k;
    }
    cout << ans << endl;
    return 0;
}