#include <iostream>
#include <fstream>
using namespace std;
ifstream f("nunta.in");
ofstream g("nunta.out");
int n, v[100000], w[100000];
void Adunare(int x[], int y[])///pbinfo 100%
{
    int i, t = 0;
    if(x[0] < y[0])
        x[0] = y[0];
    for(i = 1; i <= x[0]; i++, t /= 10)
    {
        t = x[i] + y[i] + t;
        x[i] = t % 10;
    }
    if(t)
        x[++x[0]] = t;
}
int main()
{
    f >> n;
    v[0] = w[0] = 1;
    v[1] = 1, w[1] = 2;
    if(n == 1)
        g << '1';
    else if(n == 2)
        g << '2';
    else
    {
        for(int i = 3; i <= n; i++)
        {
            Adunare(v, w);
            swap(v, w);
        }
        for(int i = w[0]; i >= 1; i--)
            g << w[i];

    }
    return 0;
}