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.Linq;
class Program
{
	static void Main(string[] args)
	{
		// スペース区切りの整数の入力
		string input = Console.ReadLine();
		var result = false;
		
		char c1 = input[0];
		char c2 = new char();
		
		foreach(var c in input){
			if(c1 != c){
				c2 = c;
			}
		}
		result = input.Count(c=>c==c1)==2 && input.Count(c=>c==c2)==2;
		
		// 出力
		var ret = "No";
		if(result){
			ret ="Yes";
		}
		Console.WriteLine(ret);
	}
}