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.IO;

public class Program
{
    static void Main(string[] args)
    {
        string s = Console.ReadLine();

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