include <bits/stdc++.h>

define ll long long

define INF 1e9

using namespace std; int main() { ll n,a=2,b=4,say=0;
cin>>n; if(n==1) { cout<<2; } else if(n==2) { cout<<4; } else { for(int i=3;i<=n;i++) { say=a+b; a=b; b=say; } cout<<say; } }