The Fibonacci numbers, commonly denoted `F(n)` form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from `0` and `1`. That is,

    F(0) = 0, F(1) = 1
    F(n) = F(n - 1) + F(n - 2), for n > 1.

Given `n`, calculate `F(n)`.

Example 1:

    Input: n = 2
    Output: 1
    Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1.

Example 2:

    Input: n = 3
    Output: 2
    Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2.

Example 3:

    Input: n = 4
    Output: 3
    Explanation: F(4) = F(3) + F(2) = 2 + 1 = 3.

Constraints:

- `0 <= n <= 30`