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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
// using System.Globalization;
// using System.IO;
using System.Linq;
// using System.Text;
// using System.Text.RegularExpressions

class Program
{
    static void Main()
    {
        string S = Console.ReadLine();
        HashSet<char> hs = new HashSet<char>(S);
        if (hs.Count() != 2) Console.WriteLine("No");
        else
        {
            int cnt = 0;
            foreach (char c in hs) if (S.Count(x => x == c) == 2) cnt++;
            if (cnt == 2) Console.WriteLine("Yes");
            else Console.WriteLine("No");
        }
        return;
    }
}