#include <bits/stdc++.h>
using namespace std;
ifstream f("nunta.in");
ofstream g("nunta.out");
int n,i;
long long x,y,z;
int main()
{
    f>>n;
    if(n<=2)
    {
        g<<n;
        return 0;
    }
    x=1;
    y=2;
    for(i=3;i<=n;i++)
       {
           z=x+y;
           x=y;
           y=z;
       }
    g<<z;
    return 0;
}