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

#define _p(...) (void)printf(__VA_ARGS__)
#define forr(x,arr) for(auto&& x:arr)
#define _overload3(_1,_2,_3,name,...) name
#define _rep2(i,n) _rep3(i,0,n)
#define _rep3(i,a,b) for(int i=int(a);i<int(b);++i)
#define rep(...) _overload3(__VA_ARGS__,_rep3,_rep2,)(__VA_ARGS__)
#define _rrep2(i,n) _rrep3(i,0,n)
#define _rrep3(i,a,b) for(int i=int(b)-1;i>=int(a);i--)
#define rrep(...) _overload3(__VA_ARGS__,_rrep3,_rrep2,)(__VA_ARGS__)
#define all(x) (x).begin(), (x).end()
#define bit(n) (1LL<<(n))
#define sz(x) ((int)(x).size())
#define fst first
#define snd second
using ll=long long;
using pii=pair<int,int>;using pll=pair<ll,ll>;using pil=pair<int,ll>;using pli=pair<ll,int>;
using vs=vector<string>;using vvs=vector<vs>;using vvvs=vector<vvs>;
using vb=vector<bool>;using vvb=vector<vb>;using vvvb=vector<vvb>;
using vi=vector<int>;using vvi=vector<vi>;using vvvi=vector<vvi>;
using vl=vector<ll>;using vvl=vector<vl>;using vvvl=vector<vvl>;
using vd=vector<double>;using vvd=vector<vd>;using vvvd=vector<vvd>;
using vpii=vector<pii>;using vvpii=vector<vpii>;using vvvpii=vector<vvpii>;
template<class T,class U>istream&operator>>(istream&is,pair<T,U>&P){is>>P.fst;is>>P.snd;return is;}
template<class T>T read(){T t;cin>>t;return t;}
template<class T,class U>ostream&operator<<(ostream&o,const pair<T,U>&p){return o<<'('<<p.fst<<", "<<p.snd<<')';}
template<class T>ostream&operator<<(ostream&o,const vector<T>&t){o<<"{";rep(i,sz(t))o<<t[i]<<",";o<<"}"<<endl;return o;}
vs s_p_l_i_t(const string&s,char c){vs v;stringstream ss(s);string x;while(getline(ss,x,c))v.emplace_back(x);return move(v);}
void e_r_r(vs::iterator it){cerr<<endl;}
template<class T,class... Args>void e_r_r(vs::iterator it,T a,Args... args){cerr<<it->substr((*it)[0]==' ',it->length())<<" = "<<a<<", ";e_r_r(++it,args...);}
#define out(args...){vs a_r_g_s=s_p_l_i_t(#args,',');e_r_r(a_r_g_s.begin(),args);}

template<class T> bool amin(T &a, const T &b) { if (b < a) { a = b; return 1; } return 0; }

void Main() {
  int n = read<int>();
  vector<ll> A(n+1);
  rep(i, n) A[i+1] = read<int>();

  const ll inf = 1e18;
  vvvl dp(n+2, vvl(n+2, vl(2, inf)));

  dp[2][n][0] = A[1];
  dp[1][n-1][0] = A[n];

  rrep(w, n) {
    rep(l, 1, n+1) {
      int r = l + w;
      if (r > n) continue;
      amin(dp[l+1][r][0], max(A[l], dp[l][r][0] + 1));
      amin(dp[l+1][r][0], max(A[l], dp[l][r][1] + (r - l + 1)));
      amin(dp[l][r-1][1], max(A[r], dp[l][r][0] + (r - l + 1)));
      amin(dp[l][r-1][1], max(A[r], dp[l][r][1] + 1));
    }
  }

  ll ans = inf;
  rep(l, 1, n+1) rep(lr, 2) ans = min(ans, dp[l][l-1][lr]);

  cout << ans << endl;
}
int main() { cin.tie(nullptr); ios::sync_with_stdio(false); Main(); return 0; }