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;
class A_FF{
	static void Main(string[] args){
		string S = Console.ReadLine();
		char a,b;
		int an,bn;
		an = 0;
		bn = 0;

		a = S[0];
		an++;
      	b = '0';

		if(a == S[1])an++;
		else{
			b = S[1];
			bn ++;
		}
		if(a == S[2])an++;
		else if(b == S[2])bn++;
		else{ 
			b = S[2];
			bn++;
		}

		if(a == S[3])an++;
		else if(b == S[3])bn++;
      
      

		if(an == 2 && bn == 2)Console.WriteLine("Yes");
		else Console.WriteLine("No");
	}
}