Problem:
Score : 100 points

Problem Statement

You are given a 4-character string S consisting of uppercase English letters.
Determine if S consists of exactly two kinds of characters which both appear twice in S.

Constraints

The length of S is 4.

S consists of uppercase English letters.

Input

Input is given from Standard Input in the following format:

S

Output

If S consists of exactly two kinds of characters which both appear twice in S, print Yes; otherwise, print No.

Sample Input 1

ASSA

Sample Output 1

Yes

S consists of A and S which both appear twice in S.

Sample Input 2

STOP

Sample Output 2

No

Sample Input 3

FFEE

Sample Output 3

Yes

Sample Input 4

FREE

Sample Output 4

No

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

using System;
using System.Linq;

namespace atcoder
{
    class Program
    {
        static void Main(string[] args)
        {
            A.FiftyFifty();
        }
    }

    class A
    {
        public static void FiftyFifty()
        {
            string s = Input.getString();

            if(s[0] == s[1] && s[1] == s[2])
            {
                Console.WriteLine("No");
            }
            else if ( s[0] == s[1]  &&  s[2] == s[3] )
            {
                Console.WriteLine("Yes");
            }
            else if( s[1] == s[2] && s[0] == s[3])
            {
                Console.WriteLine("Yes");
            }
            else if( s[0] == s[2] && s[1] == s[3])
            {
                Console.WriteLine("Yes");
            }
            else
            {
                Console.WriteLine("No");
            } 
        }
    }
  
    class Input
    {
        public static int getInt()
        {
            int i = int.Parse(Console.ReadLine());
            return i;
        }

        public static string getString()
        {
            string s = Console.ReadLine();
            return s;
        }
        public static int[] getIntArray()
        {
            string[] s = Console.ReadLine().Split(' ');
            int[] num = s.Select(x=>int.Parse(x)).ToArray();
            return num;
        }
    }

    class Output
    {
        public static void writeInt(int i)
        {
            Console.WriteLine(i);
        }
    }
}