input: Write a python function to calculate the total marks of students from a list of marks.
  assert total_marks([[75, 75, 76, 78, 80], [90, 88, 87, 85, 83], [68, 70, 79, 82, 76], [80, 79, 66, 77, 85], [90, 85, 83, 75, 73]]) == 1985
  assert total_marks([[50, 60, 70], [80, 90, 100]]) == 450
  assert total_marks([[0, 0, 0], [0, 0, 0]]) == 0
output: def total_marks(marks):
      return sum(sum(student) for student in marks)