<p>The <b>Fibonacci numbers</b>, commonly denoted <code>F(n)</code> form a sequence, called the <b>Fibonacci sequence</b>, such that each number is the sum of the two preceding ones, starting from <code>0</code> and <code>1</code>. That is,</p> <pre> F(0) = 0, F(1) = 1 F(n) = F(n - 1) + F(n - 2), for n > 1. </pre> <p>Given <code>n</code>, calculate <code>F(n)</code>.</p> <p> </p> <p><strong class="example">Example 1:</strong></p> <pre> <strong>Input:</strong> n = 2 <strong>Output:</strong> 1 <strong>Explanation:</strong> F(2) = F(1) + F(0) = 1 + 0 = 1. </pre> <p><strong class="example">Example 2:</strong></p> <pre> <strong>Input:</strong> n = 3 <strong>Output:</strong> 2 <strong>Explanation:</strong> F(3) = F(2) + F(1) = 1 + 1 = 2. </pre> <p><strong class="example">Example 3:</strong></p> <pre> <strong>Input:</strong> n = 4 <strong>Output:</strong> 3 <strong>Explanation:</strong> F(4) = F(3) + F(2) = 2 + 1 = 3. </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>0 <= n <= 30</code></li> </ul>