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 Program
{
    static void Main(string[] args)
    {
        char a, b, c, d;
        string str = Console.ReadLine();//2つ以上のスペース区切り入力の取得
        a = str[0];  //数値で受け取りたい場合は変換する
        b = str[1];
        c = str[2];
        d = str[3];
        if ((a == b && a != c && c == d) || (a == c && a != b && b == d) || (a == d && a != b && b == c))
        {

            Console.Write("Yes");
        }
        else
        {
            Console.Write("No");
        }
        string str2 = Console.ReadLine();
    }
}