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;

namespace ConsoleApp1
{
	class Program
	{
		static void Main(string[] args)
		{
			var str = Console.ReadLine();
			Dictionary<char, int> dict = new Dictionary<char, int>();
			foreach (var c in str.ToCharArray())
			{
				if (dict.ContainsKey(c))
				{
					dict[c]++;
					if (dict[c] > 2)
					{
						Console.WriteLine("No");
						return;
					}
				}
				else
				{
					dict.Add(c, 1);
				}
			}
			if (dict.Keys.Count != 2)
			{
				Console.WriteLine("No");
			}
			else
			{
				Console.WriteLine("Yes");
			}
		}
	}
}