what is the formula of fibonacci sequence code example

Example 1: fibonacci sequence

0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ...

Example 2: fibonacci sequence

def fibonacci(n):
    if n <= 0:
        return 0
    elif n == 1:
        return 1
    else:
        return fibonacci(n-1) + fibonacci(n-2)