#include <iostream>
#include<fstream>
using namespace std;
ifstream f("nunta.in");
ofstream g("nunta.out");
int main()
{int N;
f>>N;
if(N<=3)
{if(N==1 ||N==2)
    g<<2;

if(N==3)
    g<<3;}
    else
{long long a,b,c;
a=2;
b=3;
int i=N-3;
while(i>0)
    {c=a+b;
a=b;
b=c;
i--;}
g<<c;
}

    return 0;
}