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;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

class Program {
    static void Main(string[] args) {
        //入力
        string S = Console.ReadLine();

        //検証
        //出現している文字を取得する
        char S1 = S[0];
        char S2 = S[1];
        char S3 = S[2];
        char S4 = S[3];

        //取得した文字を、配列に加える
        char[] list = {S1, S2, S3, S4};
        //アルファベット順に並び替え
        Array.Sort(list);

        //出力
        if (list[0] == list[1] && list[2] == list[3] && list[0] != list[2]) {
            Console.WriteLine("Yes");
        } else {
            Console.WriteLine("No");
        }
            
    }
}