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;
using static System.Console;
using System.Text;

namespace ConsoleApp1AtCoder_Beginner_Contest_132
{
    class Program
    {
        //配列にする
        static public long[] Sarray() { return ReadLine().Split().Select(long.Parse).ToArray(); }
        //Listにする
        static public List<long> Slist() { return ReadLine().Split().Select(long.Parse).ToList(); }
        static void Main(string[] args)
        {
            var S = ReadLine();
            var d = new Dictionary<char, int>();
            foreach(var c in S)
            {
                if (!d.ContainsKey(c))
                    d[c] = 1;
                else
                    d[c]++;
            }

            if (d.Count == 2)
            {
                foreach(KeyValuePair<char, int> kv in d)
                {
                    if (kv.Value != 2)
                    {
                        WriteLine("No");
                        return;
                    }
                }
                WriteLine("Yes");
            }
            else
            {
                WriteLine("No");
            }
        }
    }
}