Language: Java

using System;
using System.Collections.Generic;
using static System.Math;
using System.Linq;
 
class DandA {
    static void Main(string[] args) {
        int n = ReadLineInt();
        string[] st = ReadLineStrArray();
        for (int i = 0;i < n; i++) {
            Console.Write(st[0].Substring(i,1) + st[1].Substring(i,1));
        }
        Console.WriteLine("");
    }
  
    static private string ReadLineStr() {return Console.ReadLine();}
    static private int ReadLineInt() {return int.Parse(Console.ReadLine());}
    static private string[] ReadLineStrArray() {return Console.ReadLine().Trim().Split();}
    static private int[] ReadLineIntArray() {return Console.ReadLine().Trim().Split().Select(a => int.Parse(a)).ToArray();}
    static private long[] ReadLineLongArray() {return Console.ReadLine().Trim().Split().Select(a => long.Parse(a)).ToArray();}
    static private string[][] ReadMLineStrArray(int arg) { return new int[arg].Select(_ => Console.ReadLine().Split()).ToArray();}
    static private int[][] ReadMLineIntArray(int arg) { return new int[arg].Select(_ => Console.ReadLine().Split().Select(int.Parse).ToArray()).ToArray();}

    static private bool IsPrime(int num) {
        if (num < 2) return false;
        if (num == 2) return true;
        else if (num % 2 == 0) return false;
        double sqrtNum = Math.Sqrt(num);
        for (int i = 3; i <= sqrtNum; i += 2) {
            if (num % i == 0) return false;
        }
        return true;
    }
}

Compiler output:
Main.java:1: error: unnamed classes are a preview feature and are disabled by default.
using System;
^
  (use --enable-preview to enable unnamed classes)
Main.java:2: error: class, interface, enum, or record expected
using System.Collections.Generic;
^
Main.java:3: error: class, interface, enum, or record expected
using static System.Math;
^
Main.java:4: error: class, interface, enum, or record expected
using System.Linq;
^
Main.java:17: error: class expected
    static private int ReadLineInt() {return int.Parse(Console.ReadLine());}
                                                 ^
Main.java:17: error: ';' expected
    static private int ReadLineInt() {return int.Parse(Console.ReadLine());}
                                                      ^
Main.java:17: error: ';' expected
    static private int ReadLineInt() {return int.Parse(Console.ReadLine());}
                                                                         ^
Main.java:19: error: illegal start of expression
    static private int[] ReadLineIntArray() {return Console.ReadLine().Trim().Split().Select(a => int.Parse(a)).ToArray();}
                                                                                                ^
Main.java:19: error: class expected
    static private int[] ReadLineIntArray() {return Console.ReadLine().Trim().Split().Select(a => int.Parse(a)).ToArray();}
                                                                                                      ^
Main.java:19: error: ')' or ',' expected
    static private int[] ReadLineIntArray() {return Console.ReadLine().Trim().Split().Select(a => int.Parse(a)).ToArray();}
                                                                                                           ^
Main.java:19: error: not a statement
    static private int[] ReadLineIntArray() {return Console.ReadLine().Trim().Split().Select(a => int.Parse(a)).ToArray();}
                                                                                                            ^
Main.java:19: error: ';' expected
    static private int[] ReadLineIntArray() {return Console.ReadLine().Trim().Split().Select(a => int.Parse(a)).ToArray();}
                                                                                                             ^
Main.java:20: error: illegal start of expression
    static private long[] ReadLineLongArray() {return Console.ReadLine().Trim().Split().Select(a => long.Parse(a)).ToArray();}
                                                                                                  ^
Main.java:20: error: class expected
    static private long[] ReadLineLongArray() {return Console.ReadLine().Trim().Split().Select(a => long.Parse(a)).ToArray();}
                                                                                                         ^
Main.java:20: error: ')' or ',' expected
    static private long[] ReadLineLongArray() {return Console.ReadLine().Trim().Split().Select(a => long.Parse(a)).ToArray();}
                                                                                                              ^
Main.java:20: error: not a statement
    static private long[] ReadLineLongArray() {return Console.ReadLine().Trim().Split().Select(a => long.Parse(a)).ToArray();}
                                                                                                               ^
Main.java:20: error: ';' expected
    static private long[] ReadLineLongArray() {return Console.ReadLine().Trim().Split().Select(a => long.Parse(a)).ToArray();}
                                                                                                                ^
Main.java:21: error: as of release 9, '_' is a keyword, and may not be used as an identifier
    static private string[][] ReadMLineStrArray(int arg) { return new int[arg].Select(_ => Console.ReadLine().Split()).ToArray();}
                                                                                      ^
Main.java:21: error: illegal start of expression
    static private string[][] ReadMLineStrArray(int arg) { return new int[arg].Select(_ => Console.ReadLine().Split()).ToArray();}
                                                                                         ^
Main.java:22: error: as of release 9, '_' is a keyword, and may not be used as an identifier
    static private int[][] ReadMLineIntArray(int arg) { return new int[arg].Select(_ => Console.ReadLine().Split().Select(int.Parse).ToArray()).ToArray();}
                                                                                   ^
Main.java:22: error: illegal start of expression
    static private int[][] ReadMLineIntArray(int arg) { return new int[arg].Select(_ => Console.ReadLine().Split().Select(int.Parse).ToArray()).ToArray();}
                                                                                      ^
Main.java:22: error: class expected
    static private int[][] ReadMLineIntArray(int arg) { return new int[arg].Select(_ => Console.ReadLine().Split().Select(int.Parse).ToArray()).ToArray();}
                                                                                                                              ^
22 errors