#include <string>
#include <vector>
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<stack>
#include<queue>
#include<cmath>
#include<algorithm>
#include<functional>
#include<list>
#include<deque>
#include<bitset>
#include<set>
#include<map>
#include<unordered_map>
#include<unordered_set>
#include<cstring>
#include<sstream>
#include<complex>
#include<iomanip>
#include<numeric>
#include<cassert>
#define X first
#define Y second
#define pb push_back
#define rep(X,Y) for (int (X) = 0;(X) < (Y);++(X))
#define reps(X,S,Y) for (int (X) = S;(X) < (Y);++(X))
#define rrep(X,Y) for (int (X) = (Y)-1;(X) >=0;--(X))
#define repe(X,Y) for ((X) = 0;(X) < (Y);++(X))
#define peat(X,Y) for (;(X) < (Y);++(X))
#define all(X) (X).begin(),(X).end()
#define rall(X) (X).rbegin(),(X).rend()
#define eb emplace_back
#define UNIQUE(X) (X).erase(unique(all(X)),(X).end())
#define Endl endl

using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
template<class T> using vv=vector<vector<T>>;
template<class T> ostream& operator<<(ostream &os, const vector<T> &t) {
os<<"{"; rep(i,t.size()) {os<<t[i]<<",";} os<<"}"<<endl; return os;}
template<class S, class T> ostream& operator<<(ostream &os, const pair<S,T> &t) { return os<<"("<<t.first<<","<<t.second<<")";}
template<class T> inline bool MX(T &l,const T &r){return l<r?l=r,1:0;}
template<class T> inline bool MN(T &l,const T &r){return l>r?l=r,1:0;}
#define out(args...){vector<string> a_r_g_s=s_p_l_i_t(#args, ','); e_r_r(a_r_g_s.begin(), args); }
vector<string> s_p_l_i_t(const string& s, char c) { vector<string> v; stringstream ss(s); string x; while(getline(ss,x,c)) v.emplace_back(x); return move(v);}
void e_r_r(vector<string>::iterator it) {}
template<typename T, typename... Args> void e_r_r(vector<string>::iterator it, T a, Args... args){ if(*it==" 1"||*it=="1") cerr<<endl; else cerr << it -> substr((*it)[0] == ' ', it -> length()) << " = " << a << ", "; e_r_r(++it, args...);}
const ll MOD=1e9+7;
const ll INF=1e15;

ll dp[2123][2123];

int solve(const vector<ll> &v){
  if(*min_element(all(v))<0) return 0;
  fill(dp[0],dp[2123],INF);
  int n=v.size();
  rep(i,n) dp[i][i]=0;
  rep(d,n)rep(i,n-d){
    if(v[i+d]<dp[i][i+d]) dp[i][i+d]=INF;
    if(v[i]<dp[i+d][i]) dp[i+d][i]=INF;
    if(i){
      MN(dp[i+d][i-1], dp[i][i+d]+d+1);
      MN(dp[i+d][i-1], dp[i+d][i]+1);
    }
    if(i+d+1<n){
      MN(dp[i][i+d+1], dp[i][i+d]+1);
      MN(dp[i][i+d+1], dp[i+d][i]+d+1);
    }
  }
  //out(v); rep(i,4){rep(j,4) cout<<dp[i][j]<<",";cout<<endl;}
  return INF>dp[0][n-1] || INF>dp[n-1][0];
}

int main(){
  // solve({2,1,1});
  // return 0;
  ios_base::sync_with_stdio(false);
  cout<<fixed<<setprecision(0);
  int n;
  cin>>n;
  vector<int> a(n);
  rep(i,n) cin>>a[i];
  if(a==vector<int>(1)){
    cout<<0<<endl;
    return 0;
  }
  int mx=*max_element(all(a));
  if(mx==0){
    cout<<n-1<<endl;
    return 0;
  }
  ll l=mx-1,r=mx+5000;
  while(r-l>1){
    ll m=(l+r)/2;
    vector<ll> v(n);
    rep(i,n) v[i]=m-a[i];
    //out(m,1);
    if(solve(v)) r=m;
    else l=m;
  }
  assert(r>=mx);
  cout<<r<<endl;
  return 0;
}