Need help writing code

Here is the source code which does this task:

using System;

namespace ConsoleApplication
{
    class Program
    {

        static void Main(string[] args)
        {

            Console.Write("Enter student id: ");
            string studentId = Console.ReadLine();

           
            Console.WriteLine("Enter three student's marks (0-100%):");

            int sum = 0;
            for (int i = 0; i < 3; i++)
            {
                string line = Console.ReadLine();
                int currentMark = int.Parse(line);
                sum += currentMark;
            }

            int average = sum / 3;
            string mark = null;
            if (average < 35)
                mark = "E";
            else if (average < 50)
                mark = "D";
            else if (average < 60)
                mark = "C";
            else if (average < 70)
                mark = "B2";
            else if (average < 80)
                mark = "B1";
            else if (average < 90)
                mark = "A2";
            else
                mark = "A1";

            Console.WriteLine("Average mark for student {0} is {1}% ({2})", studentId, average, mark);

            Console.WriteLine("Press ENTER to continue...");
            Console.ReadLine();

        }

    }
}

And here is the sample input/output:

Enter student id: 14
Enter three student's marks (0-100%):
56
29
86
Average mark for student 14 is 57% (C)
Press ENTER to continue...

Zoran