#include <bits/stdc++.h>
using namespace std;
const int N = 100005;
int t[N];
int main() {
  ios_base::sync_with_stdio(0);
  int n;
  cin >> n;
  t[0] = 0;
  t[n + 1] = 0;
  for (int i = 1; i <= n; ++i) {
    cin >> t[i];
    t[i] = min(t[i - 1] + 1, t[i]);
  }
  for (int i = n; i > 0; --i) {
    t[i] = min(t[i + 1] + 1, t[i]);
  }
  int res = 0;
  for (int i = 1; i <= n; ++i) res = max(res, t[i]);
  cout << res << "\n";
}
