Calculator using If Statement in C#

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

namespace ConsoleApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            int a;
            Console.Write("Enter Your Marks: ");
            a = Int32.Parse(Console.ReadLine());
            if (a > 100 || a < 0) //if given by user marks'a' is greater then 100 and
                Console.Write("Invalid Entery!");//less then 0 then print this line
            else if (a >= 80)  //check this if marks'a' is greater then or equal to 80 then
                Console.Write("Grade A "); //print this line
            else if (a >= 70) //check this and so on.
                Console.Write("Grade B ");
            else if (a >= 60)
                Console.Write("Grade C ");
            else if (a >= 50)
                Console.Write("Grade D ");
            else  //if not in above, means less then 50 then
                Console.Write("Fail ");  //print this line.
            Console.ReadLine();
        }
    }
}

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

namespace ConsoleApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            int a, b;
            String op;
            Console.Write("Enter first number: ");
            a = Int32.Parse(Console.ReadLine()); //user enter first integer and assigning to 'a'.
            Console.Write("Enter Operator: ");
            op = Console.ReadLine(); //user enter operator and assigning to 'op'.
            Console.Write("Enter second number: ");
            b = Int32.Parse(Console.ReadLine()); //user enter second integer and assigning to 'b'.
            if (op == "+") //if 'op' value equal to '+' then
            {
                Console.Write("Addition: Answer is{0}. ", a + b); //add and print.
            }
            else if (op == "-") //if 'op' value equal to '-' then
            {
                Console.Write("Subtraction: Answer is{0}. ", a - b); //subtract and print.
            }
            else if (op == "/")
            {
                Console.Write("Division: Answer is{0}. ", a / b);
            }
            else if (op == "*")
            {
                Console.Write("Multiplication: Answer is{0}. ", a * b);
            }
            else //if 'op' value not equal in above then
            {
                Console.Write("Invalid Entery"); //print this.
            }
            Console.ReadLine();
        }
    }
}