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


def fibonacci(n: int) -> int:
    """Return the nth Fibonacci number (0-indexed).
    
    fibonacci(0) = 0
    fibonacci(1) = 1
    fibonacci(2) = 1
    fibonacci(3) = 2
    ...
    """
    if n < 0:
        raise ValueError(f"n must be non-negative, got {n}")
    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
    
    if len(sys.argv) != 2:
        print(f"Usage: {sys.argv[0]} <n>")
        sys.exit(1)
    
    try:
        n = int(sys.argv[1])
    except ValueError:
        print(f"Error: '{sys.argv[1]}' is not a valid integer")
        sys.exit(1)
    
    result = fibonacci(n)
    print(f"fibonacci({n}) = {result}")