title: Validate input and repeat input if it was invalid
body: <p>Using Visual Studio, its an exercise, I need to add other students in, which isn't a problem. My main question is: I want the code to jump back to the input question if the value is above 100 or below 0 included, but I don't know the code that checks(validates) the user-input, any help would be very helpful and I would be incredibly grateful.</p>
  
  <pre><code>using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Text;
  
  namespace Student_Marks_For_Statement
  {
      class Program
      {
          static void Main(string[] args)
          {
              char moreData;
              double total = 0;
              double secondTotal = 0;
              for (double student1 = 0; student1 == 0; student1++)
              {
                  Console.Write("Enter mark for student 1: ");
                  student1 = Convert.ToDouble(Console.ReadLine());
                  total += student1;
  
  
  
                  Console.WriteLine("Any more data? Enter 'y' or 'n' then return");
                  moreData = Convert.ToChar(Console.ReadLine());
  
                  if (moreData == 'n')
                  {
                      ;
                  }
                  if (moreData == 'y')
                  {
                      Console.Write("Enter Value :");
                      secondTotal = Convert.ToDouble(Console.ReadLine());
                      total += secondTotal;
                  }
  
                  for (double student2 = 0; student2 == 0; student2++)
                  {
                      Console.Write("Enter mark for student 2: ");
                      student2 = Convert.ToDouble(Console.ReadLine());
                      total += student2;
                      student2++;
  
                      Console.WriteLine("Any more data? Enter 'y' or 'n' then return");
                      moreData = Convert.ToChar(Console.ReadLine());
  
                      if (moreData == 'n')
                      {
                          ;
                      }
                      if (moreData == 'y')
                      {
                          Console.Write("Enter Value :");
                          secondTotal = Convert.ToDouble(Console.ReadLine());
                          total += secondTotal;
                          student2++;
  
                      } Console.WriteLine("Total marks = : {0}", total);
  
                  }
              }
          }
      }
  }
  </code></pre>