Problem:
Score : 100 points

Problem Statement

You are given a positive integer N.
Find the minimum positive integer divisible by both 2 and N.

Constraints

1 \leq N \leq 10^9

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N

Output

Print the minimum positive integer divisible by both 2 and N.

Sample Input 1

3

Sample Output 1

6

6 is divisible by both 2 and 3.
Also, there is no positive integer less than 6 that is divisible by both 2 and 3.
Thus, the answer is 6.

Sample Input 2

10

Sample Output 2

10

Sample Input 3

999999999

Sample Output 3

1999999998

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;

class Program
{
    #region library
    static int n = 0;
    static int m = 0;
    static int r = 0;
    static int c = 0;
    static int x = 0;
    static int y = 0;
    static int[] arr;
    static string s = "";
    static string[] ss;
    static string alpha = "abcdefghijklmnopqrstuvwxyz";
    static long sum = 0;
    static long cnt = 0;
    static long max = -1;
    static long min = 2147483647;
    static bool flg = false;
    static bool flg2 = false;
    static char[,] map;
    static int[,] imap;
    static string rl()
    {
        return Console.ReadLine();
    }
    static string[] rls()
    {
        return Console.ReadLine().Split();
    }
    static void wl(object obj = null)
    {
        Console.WriteLine(obj);
    }
    static void w(object obj)
    {
        Console.Write(obj);
    }
    static int Int(string s)
    {
        return int.Parse(s);
    }
    static long Max(long a, long b)
    {
        return Math.Max(a, b);
    }
    static long Min(long a, long b)
    {
        return Math.Min(a, b);
    }
    static long Abs(long a)
    {
        return Math.Abs(a);
    }
    static void yes()
    {
        wl("YES");
    }
    static void no()
    {
        wl("NO");
    }
    static void Sum()
    {
        wl(sum);
    }
    static void Cnt()
    {
        wl(cnt);
    }
    static void setarray(int num = -1)
    {
        arr = new int[n];
        if(num == -1) ss = rls();
        for (int i = 0; i < n; i++)
        {
            if (num == -1)
                arr[i] = Int(ss[i]);
            else
                arr[i] = num;
        }
    }
    static void setmap(char def = '@')
    {
        if (r == 0 && c == 0)
        {
            ss = rls();
            r = Int(ss[0]);
            c = Int(ss[1]);
        }
        map = new char[r, c];
        for (int i = 0; i < r; i++)
        {
            if (def == '@') s = rl();
            for (int j = 0; j < c; j++)
            {
                map[i, j] = (def == '@') ? s[j] : def;
            }
        }
    }
    static void setimap(int def = -1)
    {
        if (r == 0 && c == 0)
        {
            ss = rls();
            r = Int(ss[0]);
            c = Int(ss[1]);
        }
        imap = new int[r, c];
        for (int i = 0; i < r; i++)
        {
            if (def == -1) s = rl();
            for (int j = 0; j < c; j++)
            {
                imap[i, j] = (def == -1) ? Int(s[j].ToString()) : def;
            }
        }
    }
    #endregion

    static void Main(string[] args)
    {
        n = Int(rl());
        wl(Lcm(2, n));
    }

    /* 最大公約数 */
    static long Gcd(long a, long b)
    {
        var v = new[] { a, b };
        while (v[1] != 0) { v = new[] { v[1], v[0] % v[1] }; }
        return v[0];
    }

    /* 最小公倍数(上記Gcdとセットで使うこと） */
    static long Lcm(long a, long b)
    {
        return a / Gcd(a, b) * b;
    }


}