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.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Atc_0028
{
    class Program
    {
        static void Main(string[] args)
        {
            string str = Console.ReadLine();

            for (int i = 0; i < str.Length; i++)
            {
                int index = 0;

                for (int j = 0; j < str.Length; j++)
                {
                    if (str.Substring(i, 1) == str.Substring(j, 1))
                    {
                        index++;
                    }
                }

                if (index != 2)
                {
                    Console.WriteLine("No");

                    return;
                }
            }

            Console.WriteLine("Yes");
        }
    }
}