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;
class Program
{
	static void Main(string[] args)
	{
		string str = Console.ReadLine();
		string ans = "Yes";
		if(str.Substring(0,1)==str.Substring(1,1)){
			if(str.Substring(0,1)!= str.Substring(2,1) && str.Substring(2,1)==str.Substring(3,1)){
				ans = "Yes";
			} else {
				ans = "No";
			}
		} else {
			if(str.Substring(0,1)==str.Substring(2,1) && str.Substring(1,1)==str.Substring(3,1)){
				ans = "Yes";
			} else if(str.Substring(0,1)==str.Substring(3,1) && str.Substring(1,1)==str.Substring(2,1)){
				ans = "Yes";
			} else {
				ans = "No";
			}
		}
		Console.WriteLine(ans);
	}
}