#include <bits/stdc++.h>
using namespace std;
int a[111111], b[111111], c[111111];
int main() {
  int n;
  cin >> n;
  for (int i = 1; i <= n; ++i) {
    cin >> a[i];
  }
  for (int i = 1; i <= n; ++i) {
    b[i] = min(a[i], min(b[i - 1] + 1, i));
  }
  reverse(a + 1, a + n + 1);
  for (int i = 1; i <= n; ++i) {
    c[i] = min(a[i], min(c[i - 1] + 1, i));
  }
  reverse(c + 1, c + n + 1);
  int ans = 0;
  for (int i = 1; i <= n; ++i) {
    ans = max(ans, min(b[i], c[i]));
  }
  cout << ans;
  return 0;
}
