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 AtCoder20190629
{
	class ProgramA
	{
		static void Main(string[] args)
		{
			var S = Console.ReadLine().Trim();
			A(S);
		}
		static bool JudgeA(string S)
		{
			char[] array = S.ToCharArray();
			char[] uniqueChars = array.Distinct().ToArray();
			if (uniqueChars.Length != 2) return false;
			foreach(char uc in uniqueChars)
			{
				int n = array.Count(c => c == uc);
				if (n != 2) return false;
			}
			return true;
		}
		static void A(string S)
		{
			Console.WriteLine(JudgeA(S) ? "Yes" : "No");
		}
	}
}