Problem:
Score : 100 points

Problem Statement

You are given a string S of length N consisting of A, B and C, and an integer K which is between 1 and N (inclusive).
Print the string S after lowercasing the K-th character in it.

Constraints

1 ≤ N ≤ 50

1 ≤ K ≤ N

S is a string of length N consisting of A, B and C.

Input

Input is given from Standard Input in the following format:

N K
S

Output

Print the string S after lowercasing the K-th character in it.

Sample Input 1

3 1
ABC

Sample Output 1

aBC

Sample Input 2

4 3
CABA

Sample Output 2

CAbA

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

using System;
using static System.Console;
using static System.Math;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;


namespace atcoder
{
    class Program
    {
        //メイン関数
        static void Main(string[] args)
        {
            List<int> reader = MultiScanInt(' ', 2);
            int n = reader[0];
            int k = reader[1];

            string s = scan();

            for (int i = 0; i < n; i++)
            {
                if (s.Substring(i, 1) == "A") s = s.Replace('A', 'a');
                if (s.Substring(i, 1) == "B") s = s.Replace('B', 'b');
                if (s.Substring(i, 1) == "C") s = s.Replace('C', 'c');
            }

            WriteLine(s);
        }




        #region 細々としたツール(関数)

        #region 標準入力
        //変数の入力
        static string scan() => ReadLine();
        static int scanInt() => int.Parse(ReadLine());

        //配列の入力
        static string[] MultiScan(char Split)
        {
            string[] N = ReadLine().Split(Split);
            return N;
        }
        static string[] MultiScan()
        {
            string[] N = ReadLine().Split(' ');
            return N;
        }

        static List<int> MultiScanInt(char Split)
        {
            string[] N = ReadLine().Split(Split);
            List<int> output = new List<int>();

            for (int i = 0; i < N.Length; i++)
            {
                output.Add(int.Parse(N[i]));
            }

            return output;
        }
        static List<int> MultiScanInt(char Split, int roop)
        { 
            string[] N = ReadLine().Split(Split);
            List<int> output = new List<int>();

            for (int i = 0; i < roop; i++)
            {
                output.Add(int.Parse(N[i]));
            }

            return output;
        }
        #endregion

        #region 数学関連
        public static int Gcd(int a, int b)
        {
            if (a < b)
                // 引数を入替えて自分を呼び出す
                return Gcd(b, a);
            while (b != 0)
            {
                var remainder = a % b;
                a = b;
                b = remainder;
            }
            return a;
        }//最大公約数
        public static int Lcm(int a, int b)
        {
            return a * b / Gcd(a, b);
        }//最小公倍数
        public static int PlusOneSigma(int x, int loop)
        {
            int output = 0;
            for (int i = 0; i < loop; i++)
            {
                output = output + x + i;
            }
            return output;
        }//１づつ加算のシグマ
        #endregion

        #endregion
    }
}