Language: C++

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


class Program
{
    static void Main(string[] args)
    {
        // 入力
        //var s = Console.ReadLine();
        //var n = int.Parse(Console.ReadLine());
        int[] n = Console.ReadLine().Split().Select(int.Parse).ToArray();
        var sx = n[0];
        var sy = n[1];
        var tx = n[2];
        var ty = n[3];

        StringBuilder ans = new StringBuilder("");
        //1
        //右にsx-tx
        for (int i = sx; i < tx; i++)
        {
            ans.Append("R");
        }
        //上にsy-ty
        for (int i = sy; i < ty; i++)
        {
            ans.Append("U");
        }

        //2        
        //左ににtx-sx
        for (int i = tx; i > sx; i--)
        {
            ans.Append("L");
        }
        //下にsy-ty
        for (int i = ty; i > sy; i--)
        {
            ans.Append("D");
        }

        //3
        //下に
        ans.Append("D");
        //右にsx-tx
        for (int i = sx; i <= tx; i++)
        {
            ans.Append("R");
        }
        //上にsy-ty
        for (int i = sy; i < ty; i++)
        {
            ans.Append("U");
        }
        //左に
        ans.Append("L");


        //4
        //上に
        ans.Append("U");        
        //左ににtx-sx
        for (int i = tx; i >= sx; i--)
        {
            ans.Append("L");
        }
        //下にsy-ty
        for (int i = ty; i > sy; i--)
        {
            ans.Append("D");
        }
        ans.Append("R");
        // 出力
        Console.WriteLine(ans);
    }
}

Compiler output:
a.cc:1:7: error: expected nested-name-specifier before 'System'
    1 | using System;
      |       ^~~~~~
a.cc:2:7: error: expected nested-name-specifier before 'System'
    2 | using System.Linq;
      |       ^~~~~~
a.cc:3:7: error: expected nested-name-specifier before 'System'
    3 | using System.Collections;
      |       ^~~~~~
a.cc:4:7: error: expected nested-name-specifier before 'System'
    4 | using System.Collections.Generic;
      |       ^~~~~~
a.cc:5:7: error: expected nested-name-specifier before 'System'
    5 | using System.Text;
      |       ^~~~~~
a.cc:10:22: error: 'string' has not been declared
   10 |     static void Main(string[] args)
      |                      ^~~~~~
a.cc:10:31: error: expected ',' or '...' before 'args'
   10 |     static void Main(string[] args)
      |                               ^~~~
a.cc:80:2: error: expected ';' after class definition
   80 | }
      |  ^
      |  ;
a.cc: In static member function 'static void Program::Main(int*)':
a.cc:15:12: error: structured binding declaration cannot have type 'int'
   15 |         int[] n = Console.ReadLine().Split().Select(int.Parse).ToArray();
      |            ^~
a.cc:15:12: note: type must be cv-qualified 'auto' or reference to cv-qualified 'auto'
a.cc:15:12: error: empty structured binding declaration
a.cc:15:15: error: expected initializer before 'n'
   15 |         int[] n = Console.ReadLine().Split().Select(int.Parse).ToArray();
      |               ^
a.cc:16:9: error: 'var' was not declared in this scope
   16 |         var sx = n[0];
      |         ^~~
a.cc:17:13: error: expected ';' before 'sy'
   17 |         var sy = n[1];
      |             ^~
a.cc:18:13: error: expected ';' before 'tx'
   18 |         var tx = n[2];
      |             ^~
a.cc:19:13: error: expected ';' before 'ty'
   19 |         var ty = n[3];
      |             ^~
a.cc:21:9: error: 'StringBuilder' was not declared in this scope
   21 |         StringBuilder ans = new StringBuilder("");
      |         ^~~~~~~~~~~~~
a.cc:24:22: error: 'sx' was not declared in this scope
   24 |         for (int i = sx; i < tx; i++)
      |                      ^~
a.cc:24:30: error: 'tx' was not declared in this scope
   24 |         for (int i = sx; i < tx; i++)
      |                              ^~
a.cc:26:13: error: 'ans' was not declared in this scope
   26 |             ans.Append("R");
      |             ^~~
a.cc:29:22: error: 'sy' was not declared in this scope
   29 |         for (int i = sy; i < ty; i++)
      |                      ^~
a.cc:29:30: error: 'ty' was not declared in this scope
   29 |         for (int i = sy; i < ty; i++)
      |                              ^~
a.cc:31:13: error: 'ans' was not declared in this scope
   31 |             ans.Append("U");
      |             ^~~
a.cc:36:22: error: 'tx' was not declared in this scope
   36 |         for (int i = tx; i > sx; i--)
      |                      ^~
a.cc:36:30: error: 'sx' was not declared in this scope
   36 |         for (int i = tx; i > sx; i--)
      |                              ^~
a.cc:38:13: error: 'ans' was not declared in this scope
   38 |             ans.Append("L");
      |             ^~~
a.cc:41:22: error: 'ty' was not declared in this scope
   41 |         for (int i = ty; i > sy; i--)
      |                      ^~
a.cc:41:30: error: 'sy' was not declared in this scope
   41 |         for (int i = ty; i > sy; i--)
      |                              ^~
a.cc:43:13: error: 'ans' was not declared in this scope
   43 |             ans.Append("D");
      |             ^~~
a.cc:48:9: error: 'ans' was not declared in this scope
   48 |         ans.Append("D");
      |         ^~~
a.cc:50:22: error: 'sx' was not declared in this scope
   50 |         for (int i = sx; i <= tx; i++)
      |                      ^~
a.cc:50:31: error: 'tx' was not declared in this scope
   50 |         for (int i = sx; i <= tx; i++)
      |                               ^~
a.cc:55:22: error: 'sy' was not declared in this scope
   55 |         for (int i = sy; i < ty; i++)
      |                      ^~
a.cc:55:30: error: 'ty' was not declared in this scope
   55 |         for (int i = sy; i < ty; i++)
      |                              ^~
a.cc:67:22: error: 'tx' was not declared in this scope
   67 |         for (int i = tx; i >= sx; i--)
      |                      ^~
a.cc:67:31: error: 'sx' was not declared in this scope
   67 |         for (int i = tx; i >= sx; i--)
      |                               ^~
a.cc:72:22: error: 'ty' was not declared in this scope
   72 |         for (int i = ty; i > sy; i--)
      |                      ^~
a.cc:72:30: error: 'sy' was not declared in this scope
   72 |         for (int i = ty; i > sy; i--)
      |                              ^~
a.cc:78:9: error: 'Console' was not declared in this scope
   78 |         Console.WriteLine(ans);
      |         ^~~~~~~