Or you can do it with an array and use on it the method .Max();
Like this:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
class Task
{
static void Main()
{
decimal a = decimal.Parse(Console.ReadLine());
decimal b = decimal.Parse(Console.ReadLine());
decimal c = decimal.Parse(Console.ReadLine());
decimal d = decimal.Parse(Console.ReadLine());
decimal e = decimal.Parse(Console.ReadLine());
decimal[] arrayNumbers = { a, b, c, d, e };
decimal arrayBiggest = arrayNumbers.Max();
Console.WriteLine(arrayBiggest);
}
}