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;
using System.Linq;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            int cnt = 0;
            var mondaisuu = int.Parse(Console.ReadLine());
            var mondai = Console.ReadLine().Split().Select(x => int.Parse(x)).OrderBy(x => x).ToList();

            for (int K = mondai[mondaisuu / 2 - 1]; K < mondai[mondaisuu / 2]; K++)
            {
                var arcMondai = mondai.Where(x => x >= K);
                var abcMondai = mondai.Where(x => x < K);
                if (IsCountMatch(arcMondai, abcMondai))
                {
                    cnt = arcMondai.Min() - abcMondai.Max();
                    break;
                }
            }

            Console.WriteLine(cnt);
        }

        private static bool IsCountMatch(IEnumerable<int> mondaiA, IEnumerable<int> mondaiB)
        {
            return mondaiA.Count() == mondaiB.Count();
        }

    }
}