The **Fibonacci numbers**, commonly denoted `F(n)` form a 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
```
**Example 3:**
```
Input: n = 4
Output: 3
```