#!/usr/bin/env python3
"""Simple Fibonacci calculator."""


def fib(n: int) -> int:
    """Return the nth Fibonacci number."""
    if n < 0:
        raise ValueError("n must be non-negative")
    if n == 0:
        return 0
    if n == 1:
        return 1
    a, b = 0, 1
    for _ in range(2, n + 1):
        a, b = b, a + b
    return b


if __name__ == "__main__":
    import sys
    n = int(sys.argv[1]) if len(sys.argv) > 1 else 10
    print(f"fib({n}) = {fib(n)}")