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;
class Program
{
    static void Main(string[] args)
    {
        string z = Console.ReadLine();
        char a = z[0];
        char b = z[1];
        char c = z[2];
        char d = z[3];
        if (a == b)
        {
            if(d == c)
            {
                if(a == c)
                {
                    Console.WriteLine("No");
                }
                else
                {
                    Console.WriteLine("Yes");
                }
                    

            }
            else
            {
                Console.WriteLine("No");
            }
            
        }else if (a == c)
        {
            if (b == d)
            {
                if (a == b)
                {
                    Console.WriteLine("No");
                }
                else
                {
                    Console.WriteLine("Yes");
                }
            }
            else
            {
                Console.WriteLine("No");
            }
        }else if(a == d)
        {
            if (b == c)
            {
                if (a == c)
                {
                    Console.WriteLine("No");
                }
                else
                {
                    Console.WriteLine("Yes");
                }
            }
            else
            {
                Console.WriteLine("No");
            }
        }else if(b == c)
        {
            if (a == d)
            {
                if (a == c)
                {
                    Console.WriteLine("No");
                }
                else
                {
                    Console.WriteLine("Yes");
                }
            }
            else
            {
                Console.WriteLine("No");
            }
        }else if (b == d)
        {
            if (a == c)
            {
                if (b == c)
                {
                    Console.WriteLine("No");
                }
                else
                {
                    Console.WriteLine("Yes");
                }
            }
            else
            {
                Console.WriteLine("No");
            }
        }else if(c == d)
        {
            if (a == b)
            {
                if (c == a)
                {
                    Console.WriteLine("No");
                }
                else
                {
                    Console.WriteLine("Yes");
                }
            }
            else
            {
                Console.WriteLine("No");
            }
        }
        else
        {
            Console.WriteLine("No");
        }
        
    }
}