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;

class Atcoder132
{
    public static void Main()
    {
        string input = Console.ReadLine();
        string[] vector = { input[0].ToString(), input[1].ToString(), input[2].ToString(), input[3].ToString() };

        Array.Sort(vector);

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