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.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AtCorder
{
    class Program
    {
        static int H = 0;
        static int W = 0;

        static void Main(string[] args)
        {
            var strList = new List<char> { 'A', 'T', 'C', 'G' };

            long Q = long.Parse(Console.ReadLine().Split(' ')[1]);
            string S = Console.ReadLine();

            var idx = 1;
            var acgtDict = new Dictionary<int, char>();

            char bef = 'X'; 
            //foreach (char c in S)
            //{
            //    if (bef == 'A' && c =='C')
            //    {
            //        acgtDict.Add(idx - 1, c);
            //    }
            //    bef = c;
            //    idx++;
            //}

            
            for(var i = 0; i < Q; i++)
            {

                var count = 0;
                var LR = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
                var L = LR[0];
                var R = LR[1];

                var  s = S.Substring(L - 1 , R - 1);

                bef = 'X';
                foreach (char c in s)
                {
                    if (bef == 'A' && c == 'C')
                    {
                        count++;
                    }
                    bef = c;
                    idx++;
                }

                var result = count.ToString();
                Console.WriteLine(result.ToString());

            }
        }
    }
}