C++ :

#include<cstdio>
#include<algorithm>
#include<queue>
#define N 2000010
using namespace std;
int n,a[N],maxs;
deque<int> q;
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
    {
      scanf("%d",&a[i]);
      a[i+n]=a[i];
    }
    for(int i=1;i<=2*n;i++)
    {
        while(!q.empty()&&i-q.front()>n/2) 
          q.pop_front();
        if(i>=n)
           maxs=max(maxs,a[i]+i+a[q.front()]-q.front());
          
        while(!q.empty()&&a[q.back()]-q.back()<a[i]-i) 
          q.pop_back();
        q.push_back(i);
    }
    printf("%d",maxs);
}