#include <iostream>
#include <math.h>
#include <algorithm>
#include <map>
#include <vector>
using namespace std;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define dRep(i, d, n) for (int i = d; i < (int)(n); i++)
#define forEach(arr) for (auto& it:arr)


int main(void){
    
    float a,b;
    string s;
    
    cin >> a >> b;
    // cin >> s;
    
    int n = (ceil(a / b) - 1) / 2;
    cout << n * b;
    
    
    
    
    
    
    // cout << a / (b * 2) * b;
    
}