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: Wrong Answer)

using System;

public class Test
{
	public static void Main()
	{
		// your code goes here
		
		string s = Console.ReadLine();
		string s1 = s.Substring(0,1);
		string s2 = s.Substring(1,1);
		string s3 = s.Substring(2,1);
		string s4 = s.Substring(3,1);
		
		if(s1==s2){
			if(s3==s4){Console.WriteLine("Yes");}else{Console.WriteLine("No");}
		}else if(s1==s3){
			if(s2==s4){Console.WriteLine("Yes");}else{Console.WriteLine("No");}
		}else if(s1==s4){
			if(s2==s3){Console.WriteLine("Yes");}else{Console.WriteLine("No");}
		}else{Console.WriteLine("No");}
		
	}
}