Conditions within while loop using float grades

Question

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This is what I’ve come up with so far. I need to apply beginner python skills to this program.

import math
import random
print("Enter your grades below:\n")
grades = float(input())
B = 80
A = 90
C = 70
while grades >= B and grades < A:
if grades > B:
Grades = B
B = float(input())
if grades >= B and grades < 90:
print("B is more")
elif grades < B and grades >= C:
C = float(input())
print("C is more")
if grades == -1:
print("Most of your grades are B and C")