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.Collections.Generic;
using System.Linq;
using System.Text;

public class MainClass
{
	public static void Main(string[] args)
	{
		string s = Console.ReadLine();
		Dictionary<char, int > cs = new Dictionary<char, int>(2);
		for(int i = 0; i < s.Length; i++)
		{
			char c = s[i];
			if(!cs.ContainsKey(c))
			{
				cs.Add(c, 0);
			}
			cs[c]++;
			if(cs[c]>=3)
			{
				Console.WriteLine("No");
				return;
			}
		}
		if(cs.Count == 2)
		{
			foreach (char c in cs.Keys)
			{
			
				if(cs[c]!=2)
				{
					Console.WriteLine("No");
					return;
				}
			}
					Console.WriteLine("Yes");
			
			return;
		}
		
		Console.WriteLine("No");
	}

	public static string[] Input()
	{
		return Console.ReadLine().Split(' ');
	}

	
	public static int Clamp(int val, int max, int min = 0)
	{
		return Math.Min(max, Math.Max(min, val));
	}

	
}