#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cmath>

#define rep(i,n) for(i=0; i<n; ++i)
#define in(a) cin >> a
#define out(a,b) cout << a << b
using namespace std;
using lint = long long;

int main(void){
    int i, j, n, m, k, cnt = 0;
    int l;
    in(l);
    in(k);
    while(l>k*2){
        l-=k*2;
        cnt++;
    }
    out(cnt*k,endl);
    return 0;
}