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;
using System.Threading.Tasks;

namespace ABC132
{
	class Program
	{
		static void Main(string[] args)
		{
			char[] charArray = Console.ReadLine().ToCharArray();
			Dictionary<char, int> keyValuePairs = new Dictionary<char, int>();
			for(int i = 0; i < 4; i++)
			{
				if (keyValuePairs.ContainsKey(charArray[i]))
				{
					keyValuePairs[charArray[i]]++;
				}
				else
				{
					keyValuePairs.Add(charArray[i], 1);
				}
			}

			if (keyValuePairs.Count == 2 && keyValuePairs.ContainsValue(2))
			{
				Console.WriteLine("Yes");
			}
			else
			{
				Console.WriteLine("No");
			}
		}
	}
}