#include <fstream>

using namespace std;
ifstream fin("oo.in");
ofstream fout("oo.out");
int v[100005],i,n,s,smax,viz[100005];
int main()
{
fin>>n;
for(i=1;i<=n;i++)
    fin>>v[i+1];
v[1]=v[n+1];
v[n+2]=v[2];
for(i=2;i<=n+1;i++)
{
    s=v[i]+v[i+1]-(1-viz[i-1])*v[i-1]-(1-viz[i+2])*v[i+2];
    if(s>=0)
    {
        viz[i+2]=1;
        viz[i-1]=1;
        smax+=s;
        i+=3;
    }
}
fout<<smax;
    return 0;
}