#include<bits/stdc++.h>
#define ll long long
using namespace std;
int n,a[2000001],maxx,id,ans;
int main(){
    freopen("transportt.in","r",stdin);
    freopen("transportt.out","w",stdout);
    cin>>n;
    for(int i=1;i<=n;i++){
        cin>>a[i];
        a[i+n]=a[i];
    }
    maxx=a[1]-1;
    id=1;
    for(int i=2;i<=2*n;i++){
        if(a[i-1]-i+1>maxx){
            maxx=a[i-1]-i+1;
            id=i-1;
        }else if(id<i-n/2){
            maxx=0;
            for(int j=id+1;j<i;j++){
                if(a[j]-j>maxx){
                    maxx=a[j]-j;
                    id=j;
                }
            }
        }
        ans=max(ans,a[i]+i+maxx);
    }
    cout<<ans<<endl;
}