import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Back_2748 {

    static long[] arr;

    public static void main(String[] args) throws IOException {

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(br.readLine());

        arr = new long[n + 1];

        for (int i = 0; i < n + 1; i++) {
            arr[i] = -1;
        }

        arr[0] = 0;
        arr[1] = 1;
        System.out.println(pibonacci(n));
        br.close();
    }

    private static long pibonacci(int n) {

        if (arr[n] == -1) {
            arr[n] = pibonacci(n - 2) + pibonacci(n - 1);
        }
        return arr[n];
    }
}