// Author: Rodney Shaghoulian
// Github: github.com/RodneyShag

import java.util.Scanner;

//  Time Complexity: O(n)
// Space Complexity: O(1)
public class Solution {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        scan.close();
        System.out.println(fibonacci(n));
    }

    private static int fibonacci(int n) {
        int prev = 0;
        int curr = 1;
        for (int i = 2; i <= n; i++) {
            int next = prev + curr;
            prev = curr;
            curr = next;
        }
        return curr;
    }
}