Fibonacci number in closed form expression

fib.py

import sys
import math

def fib(n):
    a = (1 + 5**(1/2)) / 2
    b = (1 - 5**(1/2)) / 2
    return (a**n - b**n)/ 5**(1/2)


if len(sys.argv) is not 2:
    print("need 1 argument: a number for fib(n).")
    exit(1)

s = sys.argv[1]
n = int(s)

print(round(fib(n)))
exit(0)