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: Runtime Error)

using System;
using System.Collections.Generic;

namespace atCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            var input = Console.ReadLine();
            int dup = 0;
            List<char> chars = new List<char>();

            foreach(char c in input)
            {
                if (chars.Contains(c) == false)
                    chars.Add(c);
                else
                    dup++;

            }

            if (dup == 2 && chars.Count == 2)
                Console.WriteLine("Yes");
            else
                Console.WriteLine("no");

            Main(null);
        }

    }
}