Demo: Fibonacci function, file upload, manual grading

The Fibonacci numbers are 1, 1, 2, 3, 5, 8, ..., where each number is the sum of the two before it. These are numbered $F_1, F_2, \ldots$, and \[ F_n = F_{n-1} + F_{n-2}. \]

Write a Python function fib that takes a number n and returns the Fibonacci number at index n, i.e., $F_n$.