#include<bits/stdc++.h>
#define endl enjoy_codeforces
using lint=long long;
int main(){
    std::cin.tie(nullptr);std::ios_base::sync_with_stdio(false);
    lint n;
    std::cin>>n;
    lint ans=1;
    for(lint i=0;n--;){
        ans+=i;
        i=ans-i;
    }
    std::cout<<ans<<'\n';
}