/* Aa^~ kokoro ga pyompyon suru n jaa^~
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*/

#include <bits/stdc++.h>
using namespace std;

using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;
using LL = long long;
using VL = vector<LL>;
using VVL = vector<VL>;
using PLL = pair<LL, LL>;
using VS = vector<string>;

#define ALL(a)  begin((a)),end((a))
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define MT make_tuple
#define SZ(a) int((a).size())
#define SORT(c) sort(ALL((c)))
#define RSORT(c) sort(RALL((c)))
#define UNIQ(c) (c).erase(unique(ALL((c))), end((c)))

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

#define FF first
#define SS second

template<class S, class T>
istream& operator>>(istream& is, pair<S,T>& p){
  return is >> p.FF >> p.SS;
}
template<class T>
istream& operator>>(istream& is, vector<T>& xs){
  for(auto& x: xs)
	is >> x;
  return is;
}
template<class S, class T>
ostream& operator<<(ostream& os, const pair<S,T>& p){
  return os << p.FF << " " << p.SS;
}
template<class T>
ostream& operator<<(ostream& os, const vector<T>& xs){
  for(unsigned int i=0;i<xs.size();++i)
	os << (i?" ":"") << xs[i];
  return os;
}
template<class T>
void maxi(T& x, T y){
  if(x < y) x = y;
}
template<class T>
void mini(T& x, T y){
  if(x > y) x = y;
}
void debug(istringstream&){}
template <char sep=',', class Head, class... Tail>
void debug(istringstream& iss, Head&& head, Tail&&... tail)
{
  string name;
  getline(iss, name, ',');
  cout << sep << name << "=" << head;
  debug(iss, forward<Tail>(tail)...);
}
#define DEBUG(...) \
  do{ \
    istringstream ss(#__VA_ARGS__); \
    debug<' '>(ss, __VA_ARGS__); \
    cout<<endl; \
  }while(0)



const double EPS = 1e-10;
const double PI  = acos(-1.0);
const LL MOD = 1e9+7;

bool split(map<int,int>& m, int i){
  auto it = m.upper_bound(i);
  if(it == begin(m)) return false;
  --it;

  if(i < it->SS){
	int r = it->SS;

	it->SS = i-1;
	if(i-1 <= it->FF)
	  m.erase(it);
	if(i+1 < r)
	  m[i+1] = r;

	return true;
  }
  return false;
}

void join(map<int,int>& m, int b, int e){
  auto it = m.upper_bound(b);
  if(it != begin(m)){
	--it;
	if(b <= it->SS){
	  it->SS = max(it->SS, e);
	}
	else{
	  it = m.insert(MP(b, e)).FF;
	}
  }
  else{
	it = m.insert(MP(b, e)).FF;
  }

  auto nit = next(it);
  while(nit != end(m)){
	if(nit->FF <= it->SS){
	  it->SS = max(it->SS, nit->SS);
	  nit = m.erase(nit);
	}
	else break;
  }
}

bool inc(map<int,int>& s, int K, int i, map<int,int>& ran){
  while(i >= 0){
	if(split(ran, i)){
	  s[i] = 1;
	  return true;
	}
	
	s[i]++;
	if(s[i] < K) return true;
	s[i] = 0;
	--i;
  }

  return false;
}

int naive(int N, VI& xs){
  int M = *max_element(ALL(xs));
  VI mem(M+1);

  int lb = 1, ub = N;
  while(ub - lb > 1){
	int mid = (lb + ub) / 2;

	bool ok = true;
	mem.assign(M+1, 0);
	FOR(i,1,N){
	  if(xs[i-1] < xs[i]){
		FOR(j,xs[i-1]+1,xs[i]+1) mem[j] = 0;
	  }
	  else if(xs[i-1] == xs[i]){
		mem[xs[i-1]]++;
		for(int l=xs[i-1];;--l){
		  if(mem[l] >= mid){
			if(l == 0){
			  ok = false;
			  break;
			}
			mem[l] = 0;
			mem[l-1]++;
		  }
		  else break;
		}
	  }
	  else{
		mem[xs[i]]++;
		for(int l=xs[i];;--l){
		  if(mem[l] >= mid){
			if(l == 0){
			  ok = false;
			  break;
			}
			mem[l] = 0;
			mem[l-1]++;
		  }
		  else break;
		}
	  }
	}

	(ok ? ub : lb) = mid;
  }

  return ub;
}

int main(){
  cin.tie(0);
  ios_base::sync_with_stdio(false);

  int N;
  cin >> N;
  VI xs(N);
  cin >> xs;
  for(auto& x: xs) --x;

  bool ok = true;
  REP(i,N-1) ok &= xs[i] < xs[i+1];
  if(ok){
	cout << 1 << endl;
	return 0;
  }

  int lb = 1, ub = N;
  while(ub - lb > 1){
	int mid = (lb + ub) / 2;

	ok = true;
	map<int,int> mp;
	int prv = xs[0];
	map<int,int> ran;
	ran[0] = 1e9;
	FOR(i,1,N){
	  if(prv < xs[i]){
		join(ran, prv+1, xs[i]+1);
	  }
	  else if(prv == xs[i]){
		if(!inc(mp, mid, prv, ran)){
		  ok = false;
		  break;
		}
	  }
	  else{
		join(ran, xs[i]+1, prv+1);
		if(!inc(mp, mid, xs[i], ran)){
		  ok = false;
		  break;
		}
	  }
	  prv = xs[i];
	}
	
	if(ok){
	  ub = mid;
	}
	else{
	  lb = mid;
	}
  }

#ifdef PYONPOI
  int ans = naive(N, xs);
  if(ans != ub){
	cerr << "expected: " << ans << ", actual: " << ub << endl;
	return 1;
  }
#endif
  cout << ub << endl;

  return 0;
}


Measured memory_bytes: 9856