#include <iostream>
#include <fstream>
using namespace std;

int dp[100005];
int v[100005];
int main()
{
    int n;
    ifstream f("oo.in");
    ofstream g("oo.out");
    f>>n;
    for(int i=1;i<=n;i++)
    {
        f>>v[i];
        if(i==1)
        {
            dp[i]=v[i];
            dp[n+1]=v[i];
        }
        else
            dp[i]=v[i]+v[i-1]+dp[i-3];
    }
    g<<dp[n];
    return 0;
}