#include<bits/stdc++.h>

using namespace std;

int N, A[2000];
int dp[2000][2001][2];


int main()
{
  cin >> N;
  for(int i = 0; i < N; i++) {
    cin >> A[i];
  }
  int low = *max_element(A, A + N), high = 2000000000;
  while(high - low > 0) {
    int mid = (0LL + low + high) / 2;
    memset(dp, -1, sizeof(dp));
    for(int i = 0; i < N; i++) dp[i][i + 1][0] = mid;
    for(int i = 0; i < N; i++) dp[i][i + 1][1] = mid;
    for(int i = 1; i < N; i++) {
      for(int j = 0; j <= N - i; j++) {
        int s = j, t = i + j;
        if(t != N) {
          dp[s][t + 1][1] = max(dp[s][t + 1][1], dp[s][t][1] - 1);
          dp[s][t + 1][1] = max(dp[s][t + 1][1], dp[s][t][0] - i);
          if(dp[s][t + 1][1] < A[t]) dp[s][t + 1][1] = -1;
        }
        if(s != 0) {
          dp[s - 1][t][0] = max(dp[s - 1][t][0], dp[s][t][1] - i);
          dp[s - 1][t][0] = max(dp[s - 1][t][0], dp[s][t][0] - 1);
          if(dp[s - 1][t][0] < A[s - 1]) dp[s - 1][t][0] = -1;
        }
      }
    }
    if(~dp[0][N][0] || ~dp[0][N][1]) high = mid;
    else low = mid + 1;
  }
  cout << low << endl;
}