#include <iostream>
#include <utility>
#include <vector>
#include <string>
#include <cmath>
#include <algorithm>
#include <functional>
#include <iomanip>
#include <map>
#include <set>
#include <queue>
using namespace std;
using ll = long long int;
#define chmax(x,y)  (x) = ((x) > (y)) ? (x) : ((x) = (y));
#define chmin(x,y)  (x) = ((x) < (y)) ? (x) : ((x) = (y));
const int INF = 1e9;
const ll INFL = 1e18;
const int Mod = 1e9 + 7;
const int dx[] = {0,1,0,-1};
const int dy[] = {1,0,-1,0};


int main(int argc, char const *argv[])
{
  int N; cin >> N;
  vector<ll> A(N); for(auto &a : A)cin >> a;
  vector<vector<vector<ll>>> dp(2,vector<vector<ll>>(N+1,vector<ll>(N+1,INFL)));
  dp[0][1][N] = A[0];
  dp[1][0][N-1] = A[N-1];
  for(int len = N;len > 0;--len)
  {
    for(int i=0;i<N;++i)
    {
      if(i+len > N)continue;
      if(i > 0)
      {
        chmin(dp[0][i+1][i+len],max(dp[0][i][i+len]+1,A[i]));
        chmin(dp[1][i][i+len-1],max(dp[0][i][i+len]+len,A[i+len-1]));
//        std::cout << dp[0][i+1][i+len] << " " << i+1 << " " << i+len << std::endl;
        std::cout << dp[1][i][i+len-1] << std::endl;
      }

      if(i+len < N)
      {
        chmin(dp[1][i][i+len-1],max(dp[1][i][i+len]+1,A[i+len-1]));
        chmin(dp[0][i+1][i+len],max(dp[1][i][i+len]+len,A[i]));
      }
    }
  }
  ll answer = INFL;
  for(int i=0;i<N;++i)
  {
    chmin(answer,min(dp[0][i][i], dp[1][i][i]));
  }
  std::cout << answer << std::endl;
}