#### Greatest Common Divisor

```"""
Greatest Common Divisor.

Wikipedia reference: https://en.wikipedia.org/wiki/Greatest_common_divisor

gcd(a, b) = gcd(a, -b) = gcd(-a, b) = gcd(-a, -b) by definition of divisibility
"""

def greatest_common_divisor(a: int, b: int) -> int:
"""
Calculate Greatest Common Divisor (GCD).
>>> greatest_common_divisor(24, 40)
8
>>> greatest_common_divisor(1, 1)
1
>>> greatest_common_divisor(1, 800)
1
>>> greatest_common_divisor(11, 37)
1
>>> greatest_common_divisor(3, 5)
1
>>> greatest_common_divisor(16, 4)
4
>>> greatest_common_divisor(-3, 9)
3
>>> greatest_common_divisor(9, -3)
3
>>> greatest_common_divisor(3, -9)
3
>>> greatest_common_divisor(-3, -9)
3
"""
return abs(b) if a == 0 else greatest_common_divisor(b % a, a)

def gcd_by_iterative(x: int, y: int) -> int:
"""
Below method is more memory efficient because it does not create additional
stack frames for recursive functions calls (as done in the above method).
>>> gcd_by_iterative(24, 40)
8
>>> greatest_common_divisor(24, 40) == gcd_by_iterative(24, 40)
True
>>> gcd_by_iterative(-3, -9)
3
>>> gcd_by_iterative(3, -9)
3
>>> gcd_by_iterative(1, -800)
1
>>> gcd_by_iterative(11, 37)
1
"""
while y:  # --> when y=0 then loop will terminate and return x as final GCD.
x, y = y, x % y
return abs(x)

def main():
"""
Call Greatest Common Divisor function.
"""
try:
nums = input("Enter two integers separated by comma (,): ").split(",")
num_1 = int(nums)
num_2 = int(nums)
print(
f"greatest_common_divisor({num_1}, {num_2}) = "
f"{greatest_common_divisor(num_1, num_2)}"
)
print(f"By iterative gcd({num_1}, {num_2}) = {gcd_by_iterative(num_1, num_2)}")
except (IndexError, UnboundLocalError, ValueError):
print("Wrong input")

if __name__ == "__main__":
main()
```  