Fibonacci - Iterative

"""
The Fibonacci sequence is a series of numbers where each number is the sum of the two numbers before it.
"""

def fibonacci(n):
    if n <= 0:
        return 0
    elif n == 1:
        return 1

    a, b = 0, 1
    for _ in range(2, n+1):
        a, b = b, a + b
    return b

print(fibonacci(20))  # output should be 6765