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;///
public class A{
    public static void Main(){
        string s = Console.ReadLine();
        var s_ = s.GroupBy(x=>x);///?
        //Console.WriteLine(s_.GetType() );
        //  System.Linq.GroupedEnumerable`3[System.Char,System.Char,System.Char]
        bool f = s_. All(x=>x.Count()==2);///?
        if (f){
            Console.WriteLine("Yes");
        }
        else{
            Console.WriteLine("No");
        }
    }
}