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# (.NET Core 3.1.201) (judge verdict: Runtime Error)

using System;
using System.Linq;

namespace AtCoderPractic0718
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            int[] nk = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();

            int N = nk[0];
            int K = nk[1];
            string S = Console.ReadLine();
            char ck = S[K];
            char[] cs = S.ToCharArray();

            for(var i =0; i<N; i++)
            {
                if(i == K-1)
                {
                    char[] cstmp =  cs[i].ToString().ToLower().ToCharArray();
                    cs[i] = cstmp[0];
                }
            }

            string ans = new string(cs);

            Console.WriteLine(ans);
            //k=1 hk=5 　k=2 h2=10 
        }
    }
}