#include <iostream>
#include <fstream>
using namespace std;

ifstream f ("nunta.in");
ofstream g ("nunta.out");

short N, s1[101], s2[101];

void add (short A[], short B[]) {
    int i, t = 0;
    for (i = 1; i <= A[0] || i <= B[0] || t; i++, t /= 10)
        A[i] = (t += A[i] + B[i]) % 10;
    A[0] = i - 1;
}

void afisare (short A[]) {
    for (int i = A[0]; i >= 1; i--)
        g << A[i];
}

int main() {
    f >> N;
    s1[0] = 1;
    s1[1] = 1;
    s2[0] = 1;
    s2[1] = 2;
    if (N == 1) g << 1;
    else if (N == 2) g << 2;
    else {
        for (int i = 3; i <= N; i++)
            if (i % 2 == 1)
                add (s1, s2);
            else add (s2, s1);
        if (N % 2 == 1) afisare (s1);
        else afisare (s2);


    }
    return 0;
}