#include <bits/stdc++.h>
using namespace std;
long long int mod_add(long long int a, long long int b, long long int m) {
  a = a % m;
  b = b % m;
  return (((a + b) % m) + m) % m;
}
long long int mod_mul(long long int a, long long int b, long long int m) {
  a = a % m;
  b = b % m;
  return (((a * b) % m) + m) % m;
}
long long int mod_sub(long long int a, long long int b, long long int m) {
  a = a % m;
  b = b % m;
  return (((a - b) % m) + m) % m;
}
bool sortbysec(const pair<int, int> &a, const pair<int, int> &b) {
  return (a.second < b.second);
}
bool prime[1000000 + 9];
void seive() {
  long long int N = 1e6 + 1;
  memset(prime, true, sizeof(prime));
  prime[0] = false;
  prime[1] = false;
  for (long long int i = 2; i * i <= N; i++) {
    if (prime[i]) {
      for (long long int j = i * i; j <= N; j += i) {
        prime[j] = false;
      }
    }
  }
}
bool isPowerOfTwo(int x) { return (x && !(x & (x - 1))); }
bool is_prime(int n) {
  if (n <= 1) return false;
  if (n <= 3) return true;
  if (n % 2 == 0 || n % 3 == 0) return false;
  for (int i = 5; i * i <= n; i = i + 6)
    if (n % i == 0 || n % (i + 2) == 0) return false;
  return true;
}
int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, -1, 1};
void hemant() {
  int n;
  cin >> n;
  vector<long long int> ar(n + 1);
  for (int i = 1; i <= n; i++) {
    cin >> ar[i];
  }
  vector<long long int> curr(n + 1);
  curr[n] = 1;
  for (int i = n - 1; i >= 1; i--) {
    curr[i] = ((ar[i] > curr[i + 1] + 1) ? curr[i + 1] + 1 : ar[i]);
  }
  long long int ans = 1;
  curr[1] = 1;
  for (int i = 2; i <= n; i++) {
    curr[i] = ((curr[i] > ((curr[i - 1] + 1 > ar[i]) ? ar[i] : curr[i - 1] + 1))
                   ? ((curr[i - 1] + 1 > ar[i]) ? ar[i] : curr[i - 1] + 1)
                   : curr[i]);
    ans = (curr[i] < ans ? ans : curr[i]);
  }
  cout << ans << "\n";
}
int main() {
  int t = 1;
  for (int i = 0; i < t; i++) {
    hemant();
  }
  return 0;
}
