public void CalculateAverageGrade()
{
    double gradeSum = 0;
    double creditsSum = 0;

    foreach (SemesterLecture semesterLecture in selectedSemester.semesterLectures)
    {
        try{
            double grade = Double.Parse(semesterLecture.grade) * semesterLecture.lecture.credits;
            gradeSum += grade;
            creditsSum += semesterLecture.lecture.credits;
        }
        catch (Exception e){}
    }

    if (Double.IsNaN(Math.Round((gradeSum / creditsSum), 1, MidpointRounding.AwayFromZero)))
    {
        semesterGrade.text = "-";
    }
    else
    {
        semesterGrade.text = Math.Round((gradeSum / creditsSum), 1, MidpointRounding.AwayFromZero) + "";
    }
}