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;
using System.Collections.Generic;

namespace Algorithm
{
    class Program
    {
        static void Main(string[] args)
        {
            var S = Console.ReadLine();

            var dict = new Dictionary<char, int>();
            for (var i = 0; i < S.Length; i++)
            {
                if (dict.ContainsKey(S[i]))
                    dict[S[i]]++;
                else
                    dict[S[i]] = 1;
            }

            if (dict.Count == 2 && !dict.Any(z => z.Value != 2))
                Console.WriteLine("Yes");
            else
                Console.WriteLine("No");
        }
    }
}