Classifications of Marks by Grades

I have data of a Subject Marks in a column of marks.csv file as given below.

    Students Marks
0   Santosh 90
1   Mahesh  35
2   Suresh  15
3   Ganesh  45
4   Ramesh  60
5   Supriya 25
6   Ritesh  50
7   Pooja   95

I want to classify this marks as per grades:

Marks > 80 (Distinguished);
Marks > = 50 & Marks < 80 (First Class);
Marks > = 35 & Marks < 50 (Second Class);
Marks  < 35 (Failed).

In result, I need output in same file .csv as like given below.

    Student Marks Grades
0   Santosh 90  Distinction
1   Mahesh  35  Secnd Class
2   Suresh  15  Failed
3   Ganesh  45  Secnd Class
4   Ramesh  60  First Class
5   Supriya 25  Failed
6   Ritesh  50  First Class
7   Pooja   95  Distinction

Any suggestions is helpful.