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;//list用
using System.Text;//StringBuilder用
using System.Collections;
using System.Linq;
class Program
{
static int result;
static List<int> s=new List<int>();
  static List<int> t=new List<int>();
  static int N;
  
    static void Main()
    {
      string st=Console.ReadLine();
      if(st[0]==st[1] && st[2]==st[3] && st[1]!=st[2]){      Console.WriteLine("Yes");return;}
      if(st[0]==st[2] && st[1]==st[3] && st[0]!=st[1]){ Console.WriteLine("Yes");return;}
      if(st[0]==st[3] && st[1]==st[2] && st[1]!=st[3]){ Console.WriteLine("Yes");return;}

      Console.WriteLine("No");
      
      
                    //Console.WriteLine(result.ToString());
    }

static void Read()
{
  string[] tmp=Console.ReadLine().Split(' ');
for(int i=0;i<N;i++){s.Add(int.Parse(tmp[i]));}
}
}