#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 h=0,y=0;
    while(1) {
        if(h+k+y>=l) break;
        h+=k;
        y+=k;
    }
    if(l<=2*k) {cout<<'0'<<endl; return 0;}
    cout<<y<<endl;
    return 0; 
}