#include <iostream>
using namespace std;
int d[20] = {0};
int bi(int n){
if(d[n-1]){
return d[n-1];
}
else{
return d[n-1] = bi(n-1)+bi(n-2);
}
}
int main(){
int n;
d[0] = 2;
d[1] = 3;
cin >> n;
cout << bi(n);
}