#include<bits/stdc++.h>
using namespace std;
#define int long long
const int N=5e5+7;
int n,x[N],mx=0;
struct node{
	int d;
	node *nxt,*pre;
	node(){d=-1;pre=nullptr;nxt=nullptr;}
	node(int t){d=t;pre=nullptr;nxt=nullptr;}
}*head,*tail,*p;
signed main(){
//	freopen("ex.in","r",stdin);
//	freopen("ex.ans","w",stdout);
//	system("fc ex.out ex.ans");
//	return 0;
	tail=head=new node;
	cin>>n;
	for(int i=1;i<=n;i++)cin>>x[i],p=new node(x[i]),tail->nxt=p,p->pre=tail,tail=p;
	p=head->nxt;
	while(p->nxt!=nullptr){
		mx=max(mx,p->d);
		if(p!=head&&p->pre->d==p->d){
			p=p->pre;
			p->d++;
			p->nxt=p->nxt->nxt,p->nxt->pre=p;
			mx=max(mx,p->d);
			continue;
		} 
		p=p->nxt;
	}
	tail=head=new node;
	for(int i=n;i>=1;i--)p=new node(x[i]),tail->nxt=p,p->pre=tail,tail=p;
	p=head;
	while(p->nxt!=nullptr){
		mx=max(mx,p->d);
		if(p!=head&&p->pre->d==p->d){
			p=p->pre;
			p->d++;
			p->nxt=p->nxt->nxt,p->nxt->pre=p;
			mx=max(mx,p->d);
			continue;
		} 
		p=p->nxt;
	} 
	cout<<mx<<'\n';
	return 0;
}