Not understanding how to make my for loop and if statement work

I have this, but I'm very lost in how I can get the final GPA to print out, I've tried various ways but have not been able to do it successfully. This is what I have:

Scanner input = new Scanner(System.in);

    System.out.println("How many grades are you putting? ");
    int length = input.nextInt();
    input.nextLine();

    String[] gradesArray = new String[length];

    for(int i = 0; i < gradesArray.length; i++)
    {
        System.out.println("Enter grade (include + or -) ");
        gradesArray[i] = input.nextLine();

        double points = 0.0;
        if(gradesArray[i].equalsIgnoreCase("A+") || gradesArray[i].equalsIgnoreCase("A"))
        {
           points += 4;

        }
        else if(gradesArray[i].equalsIgnoreCase("A-"))
        {
            points+= 3.7;

        }
        else if(gradesArray[i].equalsIgnoreCase("B+"))
        {
            points += 3.3;

        }
        else if(gradesArray[i].equalsIgnoreCase("B"))
        {
            points += 3.0;

        }
        else if(gradesArray[i].equalsIgnoreCase("B-"))
        {
            points += 2.7;

        }
        else if(gradesArray[i].equalsIgnoreCase("C+"))
        {
            points += 2.3;

        }
        else if(gradesArray[i].equalsIgnoreCase("C"))
        {
            points += 2.0;

        }
        else if(gradesArray[i].equalsIgnoreCase("D"))
        {
            points += 1.0;

        }
        else if(gradesArray[i].equalsIgnoreCase("F"))
        {
            points += 0.0;

        }
        else
        {
            System.out.println("Invalid grade");
        }
        System.out.println("GPA: " + points / gradesArray.length);
    }

I'm guessing the GPA does not print out properly because after the condition matches the grade, it then goes right down to print, right? And also, how can I do it so if they enter an invalid grade, it makes the user start over.