#include <bits/stdc++.h>
using namespace std;
int arr[100005];
int lfl[100005];
int rfl[100005];
int main() {
  int n;
  cin >> n;
  int i, j;
  for (i = 1; i <= n; i++) {
    cin >> arr[i];
  }
  for (i = 1; i <= n; i++) {
    lfl[i] = min(lfl[i - 1] + 1, arr[i]);
  }
  long long ans = INT_MIN;
  for (i = n; i >= 1; i--) {
    rfl[i] = min(rfl[i + 1] + 1, arr[i]);
    ans = (long long)max(ans, (long long)min(lfl[i], rfl[i]));
  }
  cout << ans << endl;
  return 0;
}
