#include<bits/stdc++.h> 
#define rint register int
using namespace std;
set<pair<int,int> >st;
int a[222222],flag,n;
inline void pluss(int x,int now)
{
	pair<int,int>tmp;
	if(x<1) flag=-1;
	tmp=*st.lower_bound(make_pair(x,0));
	if(tmp.first!=x) st.insert(make_pair(x,0)),tmp=make_pair(x,0);
	else
	{
		st.erase(tmp);tmp.second++;
		if(tmp.second>=now) pluss(x-1,now);
		else st.insert(tmp);
	}
	while(1)
	{
		if(st.upper_bound(tmp)==st.end()) break;
		tmp=*st.upper_bound(tmp); st.erase(tmp);
	}
}
inline int check(int x)
{
	st.clear(); flag=0;
	for(rint i=1;i<=n;i++) if(a[i]<=a[i-1]) pluss(a[i],x-1);
	if(flag==-1)
		return 0;
	return 1;
}
inline int erf(int l,int r)
{
	while(l<=r)
	{
		int mid=l+r>>1;
		if(check(mid)) r=mid-1;
		else l=mid+1;
	}return l;
}
int main()
{
	cin>>n;int flag=1;
	for(rint i=1;i<=n;i++)
	{
		cin>>a[i];
		flag&=(a[i]>a[i-1]);
	}int ans;
	if(flag==1){cout<<1;return 0;}
	ans=erf(2,n);cout<<ans;
 	return 0;
}
