#include <bits/stdc++.h>
using namespace std;
int a[123456];
int main() {
  int n, ans = 0;
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  a[n + 1] = a[0] = 0;
  for (int i = 1; i <= n; i++)
    a[i] = min(a[i], min(a[i - 1] + 1, a[i + 1] + 1));
  for (int i = n; i > 0; i--) {
    a[i] = min(a[i], min(a[i - 1] + 1, a[i + 1] + 1));
    ans = max(ans, a[i]);
  }
  cout << ans << endl;
  return 0;
}
