#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long n, x;
  cin >> n;
  long long a[n + 1];
  for (long long i = 0; i < n; i++) {
    cin >> a[i + 1];
  }
  long long dp[n + 2];
  dp[0] = 0;
  dp[n + 1] = 0;
  for (long long i = 1; i <= n; i++) dp[i] = min(dp[i - 1] + 1, a[i]);
  for (long long i = n; i >= 1; i--) dp[i] = min(dp[i + 1] + 1, dp[i]);
  long long mx = dp[1];
  for (long long i = 2; i <= n; i++) mx = max(mx, dp[i]);
  cout << mx;
  return;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ;
  long long t;
  t = 1;
  while (t--) solve();
  return 0;
}
