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.Collections.Generic;
using System.Linq;

class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("ASSA");
            //Yes
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("STOP");
            //No
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("FFEE");
            //Yes
        }
        else if (InputPattern == "Input4") {
            WillReturn.Add("FREE");
            //No
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        List<string> InputList = GetInputList();
        string s = InputList[0];

        if (s.ToCharArray().Distinct().Count() != 2) {
            Console.WriteLine("No");
            return;
        }

        int[] CntArr = s.ToCharArray().GroupBy(X => X).Select(X => X.Count()).ToArray();
        if (CntArr[0] != 2) {
            Console.WriteLine("No");
        }
        else if (CntArr[1] != 2) {
            Console.WriteLine("No");
        }
        else {
            Console.WriteLine("Yes");
        }
    }
}