<human>:  greatest common divisor function in.
<bot>: Here is a Python function to the greatest common divisor in the Python code

import math

def gcd(a, b):
  """Returns the greatest common divisor of a and b."""
  if a == 0:
    return b
  if b == 0:
    return a
  return gcd(b % a, a)

# Enter the numbers
a = 10
b = 15

# Calling the function
print((gcd(a, b)))

#Example Output
# 5

#Remember to inspect code for bugs.