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 static System.Console;

public class Hello
{
    public static void Main()
    {
        string l = ReadLine();
        bool b = false;
        if (l[0] == l[1] && l[2] == l[3]) b = true;
        if (l[0] == l[2] && l[1] == l[3]) b = true;
        if (l[0] == l[3] && l[2] == l[1]) b = true;
      	if (l[0] == l[1] && l[1] == l[2] && l[2] == l[3]) b =false;

        if (b) WriteLine("Yes");
        else WriteLine("No");

    }
}