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;

namespace question1
{
    class Program
    {
        static void Main(string[] args)
        {
            var p = new Program();
            p.Run();
        }

        void Run(){
            var s = Console.ReadLine();
            var chars = s.ToCharArray().OrderBy(c=>c).ToList();

            if( chars[0] == chars[1] && chars[2] == chars[3] && chars[0] != chars[2]){
                Console.WriteLine("Yes");
            }else{
                Console.WriteLine("No");
            }
        }
    }
}