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;
class fifty{
  public static void Main(){
    String str = Console.ReadLine();
    checklist c = new checklist();
    int kind=0,count=0,p=0;
    char[] list = new char[str.Length+1];
    for(int i=0; i<str.Length; i++){
      if(!c.check(list,str[i])){
        kind++;
        list[p] = str[i];
        p++;
      }
    }
    for(int i=0; i<str.Length; i++){
      if(str[0].Equals(str[i])){
        count++;
      }
    }
    if(count == 2 && kind == 2){
      Console.WriteLine("Yes");
    }
    else{
      Console.WriteLine("No");
    }
  }
}

class checklist{
  public bool check(char[] list, char c){
    for(int i=0; i<list.Length; i++){
      if(list[i].Equals(c)){
        return true;
      }
    }
    return false;
  }
}