#include<cstdio> #include<algorithm> #include<cmath> #include<iostream> using namespace std; long long dp[55]={1}; long long S[55]={0}; int N; int main(void){ cin>>N; if(N<=3){cout<<N<<endl;} else{ dp[2]=1; dp[3]=1; S[3]=3; for(int i=4;i<=N;i++){ dp[i]=dp[i-1]+dp[i-2]; S[i]=S[i-1]+dp[i]; } cout<<S[N]<<endl; } return 0; }