using LIB; using System; using System.Linq; using System.Text; using System.Collections.Generic; class Program { public static IO IO = new IO(); static void Main(string[] args) { int[] n = IO.INT(' '); int o = n[0] / 2 / n[1]; if (n[0] == n[1] * 2 * o) { o--; } o *= n[1]; IO.WRITE(o); IO.FLUSH(); } } namespace LIB { public class IO { private const int WMAX = 1000; private StringBuilder S = new StringBuilder(); private T R(Func f) { return f(Console.ReadLine()); } private T[] R(Func f, char c) { return STRING().Split(c).Select(f).ToArray(); } private T[] R(Func f, int l) { T[] r = new T[l]; for (int i = 0; i < l; i++) { r[i] = R(f); } return r; } private T[][] R(Func f, int l, char c) { T[][] r = new T[l][]; for (int i = 0; i < l; i++) { r[i] = R(f, c); } return r; } private void W(Func f, T v, bool lf = true) { S.Append(f(v)); if (lf == true) { S.Append('\n'); } if (S.Length >= WMAX) { FLUSH(); } } public string STRING() { return R(s => s); } public string[] STRING(char c) { return R(s => s, c); } public string[] STRING(int l) { return R(s => s, l); } public string[][] STRING(int l, char c) { return R(s => s, l, c); } public int INT() { return R(int.Parse); } public int[] INT(char c) { return R(int.Parse, c); } public int[] INT(int l) { return R(int.Parse, l); } public int[][] INT(int l, char c) { return R(int.Parse, l, c); } public long LONG() { return R(long.Parse); } public long[] LONG(char c) { return R(long.Parse, c); } public long[] LONG(int l) { return R(long.Parse, l); } public long[][] LONG(int l, char c) { return R(long.Parse, l, c); } public double DOUBLE() { return R(double.Parse); } public double[] DOUBLE(char c) { return R(double.Parse, c); } public double[] DOUBLE(int l) { return R(double.Parse, l); } public double[][] DOUBLE(int l, char c) { return R(double.Parse, l, c); } public void WRITE(string s, bool lf = true) { W(v => v, s, lf); } public void WRITE(int s, bool lf = true) { W(v => v.ToString(), s, lf); } public void WRITE(long s, bool lf = true) { W(v => v.ToString(), s, lf); } public void WRITE(double s, bool lf = true) { W(v => v.ToString(), s, lf); } public void FLUSH() { Console.Write(S); S.Length = 0; } } }