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;

namespace ABC132
{
    class Program
    {
        static int Main(string[] args)
        {
            problemA();
            return 0;
        }

        static void problemA()
        {
            string input = Console.ReadLine();
            string ans = "No";

            if (input[0] == input[1] && input[1] != input[2] && input[2] == input[3])
            {
                ans = "Yes";
            }
            else if (input[0] == input[2] && input[0] != input[1] && input[1] == input[3])
            {
                ans = "Yes";
            }
            else if (input[0] == input[3] && input[0] != input[1] && input[1] == input[2])
            {
                ans = "Yes";
            }
            

            Console.WriteLine(ans);
        }
    }
}