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

#define endl '\n'
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define spa << " " <<
#define test cout<<"test"<<endl;
#define fi first
#define se second
#define MP make_pair
#define PB push_back
#define EB emplace_back
#define rep(i,n,m) for(ll i = n; i < (ll)(m); i++)
#define rrep(i,n,m) for(ll i = m - 1; i >= (ll)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD = 1e9+7;
//const ll MOD = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template<typename T>
void chmin(T &a,T b){if(a>b)a=b;}
template<typename T>
void chmax(T &a,T b){if(a<b)a=b;}
void pmod(ll &a,ll b){a=(a+b)%MOD;}
void pmod(ll &a,ll b,ll c){a=(b+c)%MOD;}
void qmod(ll &a,ll b){a=(a*b)%MOD;}
void qmod(ll &a,ll b,ll c){a=(b*c)%MOD;}
ll median(ll a,ll b, ll c){return a+b+c-max({a,b,c})-min({a,b,c});}
void ans1(bool x){if(x) cout<<"Yes"<<endl;else cout<<"No"<<endl;}
void ans2(bool x){if(x) cout<<"YES"<<endl;else cout<<"NO"<<endl;}
void ans3(bool x){if(x) cout<<"Yay!"<<endl;else cout<<":("<<endl;}
template<typename T1,typename T2>
void ans(bool x,T1 y,T2 z){if(x)cout<<y<<endl;else cout<<z<<endl;}  
template<typename T>
void debug(vector<vector<T>>&v,ll h,ll w){for(ll i=0;i<h;i++)
{cout<<v[i][0];for(ll j=1;j<w;j++)cout spa v[i][j];cout<<endl;}};
void debug(vector<string>&v,ll h,ll w){for(ll i=0;i<h;i++)
{for(ll j=0;j<w;j++)cout<<v[i][j];cout<<endl;}};
template<typename T>
void debug(vector<T>&v,ll n){if(n!=0)cout<<v[0];
for(ll i=1;i<n;i++)cout spa v[i];cout<<endl;};
template<typename T>
vector<vector<T>>vec(ll x, ll y, T w){
  vector<vector<T>>v(x,vector<T>(y,w));return v;}
ll gcd(ll x,ll y){ll r;while(y!=0&&(r=x%y)!=0){x=y;y=r;}return y==0?x:y;}
vector<ll>dx={1,0,-1,0,1,1,-1,-1};
vector<ll>dy={0,1,0,-1,1,-1,1,-1};
template<typename T>
vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v(ts...))>(a,make_v(ts...));
}

int main(){
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  //cout<<fixed<<setprecision(10);
  ll res=0,buf=0;
  bool judge = true;
  ll n;cin>>n;
  vector<ll>a(n);
  rep(i,0,n)cin>>a[i];
  ll l=0,r=n;
  while(r-l>=2){
    ll mid=(l+r)/2;
    if(mid==1){
      judge=true;
      rep(i,0,n-1)if(a[i]>=a[i+1])judge=false;
      if(judge)r=mid;
      else l=mid;
    }
    else{
      stack<P>st;
      st.emplace(0,-INF);
      rep(i,0,n){
        ll cnt=1;
        //cout<<st.top().fi spa st.top().se spa mid<<endl;
        while(st.top().fi>a[i]){
          cnt++;
          st.pop();
        }
        if(cnt>=2)cnt=2;
        auto p=st.top();
        if(p.fi==a[i]){
          if(p.se<mid){
            p.se++;
            st.pop();
            st.push(p);
          }
          else{
            buf=a[i]+1;
            while(p.fi+1==buf&&p.se==mid){
              st.pop();
              p=st.top();
              buf--;
            }
            if(buf==p.fi+1){
              st.pop();
              st.emplace(p.fi,p.se+1);
            }
            else{
              st.emplace(buf-1,2);
              st.emplace(buf,1);
            }
          }
        }
        else st.emplace(a[i],cnt);

        /*stack<P> tmp;
        cout<<mid<<endl;
        while(!st.empty()){
          cout<<st.top().fi spa st.top().se<<endl;
          tmp.push(st.top());
          st.pop();
        }
        while(!tmp.empty()){
          st.push(tmp.top());tmp.pop();
        }*/
      }
      while(st.size()!=1)st.pop();
      if(st.top().se!=-INF)l=mid;
      else r=mid;
    }
  }
  cout<<r<<endl;
  return 0;
}

Measured memory_bytes: 5172