#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i < (int)(n); ++i)
#define rep2(i, s, n) for (int i = (s); i < (int)(n); ++i)
#define fore(i,a) for(auto &i:a)
#define INF32 2147483647
#define INF64 9223372036854775807
using vi=vector<int>;
using vii=vector<vector<int>>;
template<class T> inline bool chmin(T& a, T b) {if (a > b) {a = b;return true;}return false;}
template<class T> inline bool chmax(T& a, T b) {if (a < b) {a = b;return true;}return false;}



int main() {
    long long l,k;
    cin>>l>>k;
    long long y;
    if(l<=2*k) y=0;
    else y=(l/k)/2*k;
    cout<<y<<endl;
    return 0; 
}