#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}

//INSERT ABOVE HERE
signed main(){
  Int n;
  cin>>n;
  Int dp[2][100];
  memset(dp,0,sizeof(dp));
  dp[0][0]=1;
  for(Int i=0;i<=n;i++){
    //cout<<i<<":"<<dp[0][i]<<" "<<dp[1][i]<<endl;
    dp[0][i+1]+=dp[0][i];
    dp[1][i+2]+=dp[0][i];
    
    dp[0][i+1]+=dp[1][i];
    dp[1][i+2]+=dp[1][i];
  }
  cout<<dp[0][n]+dp[1][n]<<endl;
  return 0;
}