#include <bits/stdc++.h>
using namespace std;
template <typename T> using min_priority_queue = priority_queue<T,vector<T>,greater<T>>;

template<typename T>
void printv(vector<T> &v){
    bool b = false;
    for(auto i : v){
        if(b) cout << " ";
        else b = true;
        cout << i;
    }
    cout << endl;
}

int64_t rand64(){
    int64_t l = rand();
    int64_t r = rand();
    return (l<<31)+r;
}

template <typename T>
bool chmin(T &a, const T& b) {
    if (a > b) {
        a = b;  // aをbで更新
        return true;
    }
    return false;
}

template <typename T>
bool chmax(T &a, const T& b) {
    if (a < b) {
        a = b;  // aをbで更新
        return true;
    }
    return false;
}

bool yn(bool b){
    if(b) cout << "Yes" << endl;
    else cout << "No" << endl;
    return b;
}

bool debug;
bool randomInput;
bool debugOutput;
int numOfTestCase;

using ans_type = int;

void input(){
    if(numOfTestCase > 1){
    }

    if(randomInput){
        
    }
    else{
    }
    return;
}

void output_input(){
    ;
}

ans_type calc(){
    int L, K;
    cin >> L >> K;
    int cnt = (L-1)/(2*K);
    cout << cnt*K << endl;
    return ans_type();
}

ans_type calc_simple(){
    return ans_type();
}

void output(ans_type ans){
    return;
}

int main(){
    debug = 0;
    randomInput = 0;
    debugOutput = 0;
    numOfTestCase = 1;
    srand(time(NULL));
    cout << fixed << setprecision(12);
    
    if(numOfTestCase == 0) cin >> numOfTestCase;

    if(debug){
        for(int i = 0; i < numOfTestCase; i++){
            if(debugOutput) cout << string(16, '-') << endl;
            input();
            ans_type ans = calc();
            ans_type ansSimple = calc_simple();
            if(ans != ansSimple){
                output_input();
                output(ans);
                output(ansSimple);
            }
        }
    }
    else{
        for(int i = 0; i < numOfTestCase; i++){
            input();
            output(calc());
        }
    }

    return 0;
}