def fib( n ):
    """
    fib( n ). 
    This method calculate the nth Fibonacci number.

    Parameters:
    n(int): the nth Fibonacci number in the Fibonacci sequence

    Returns:
    f: nth Fibonacci number.
        
    """
    if n < 2:
        return 1
    else:
        f = fib(n-1) + fib(n-2)
    return f