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: Runtime Error)

using System;
using System.Linq;

class Plogram
{
    static void Main()
    {
        string str = Console.ReadLine();

        int num = 0;
        int cont = 1;
        var contmax = new int[str.Length - 1];
        string acgt0;
        string acgt1;

        for (int i = 0; i < str.Length - 1; i++)
        {
            acgt0 = str.Substring(i, 1);
            acgt1 = str.Substring(i + 1, 1);

            if (acgt0 == "A" || acgt0 == "C" || acgt0 == "G" || acgt0 == "T")
            {
                num++;
                if (acgt1 == "A" || acgt1 == "C" || acgt1 == "G" || acgt1 == "T")
                {
                    cont++;
                    if (i == str.Length - 1)
                    {
                        num++;
                    }
                }
                else
                {
                    cont = 1;
                }
                contmax[i] = cont;
            }
        }
        Console.WriteLine((contmax.Max(x=>x) < 2) ? num : contmax.Max(x=>x));
    }
}