Problem:
Score : 100 points

Problem Statement

On the Planet AtCoder, there are four types of bases: A, C, G and T. A bonds with T, and C bonds with G.

You are given a letter b as input, which is A, C, G or T. Write a program that prints the letter representing the base that bonds with the base b.

Constraints

b is one of the letters A, C, G and T.

Input

Input is given from Standard Input in the following format:

b

Output

Print the letter representing the base that bonds with the base b.

Sample Input 1

A

Sample Output 1

T

Sample Input 2

G

Sample Output 2

C

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        var S = Console.ReadLine();
        var subStrs = new HashSet<char>() { 'A', 'C', 'G', 'T' };

        int maxLen = 0;
        for (int i = 0; i < S.Length; i++)
        {
            for(int subLen = S.Length - i; i < subLen ; subLen--)
            {
                var sub = S.Substring(i, subLen);
                int count = 0;
                foreach (var ch in sub)
                {
                    if (subStrs.Contains(ch))
                    {
                        count++;
                        if(maxLen < count)
                        {
                            maxLen = count;
                        }
                    }
                    else
                    {
                        count = 0;
                        continue;
                    }
                }

            }
        }

        Console.WriteLine(maxLen);
    }
}