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 s = Console.ReadLine();
    HashSet<char> hs = new HashSet<char>();

    for (int i = 0; i < s.Length; ++i) {
      hs.Add(s[i]);
    }

    if (hs.Count == 2) {
      Console.WriteLine("Yes");
    } else {
      Console.WriteLine("No");
    }
  }
}