#include <bits/stdc++.h>
using namespace std;
template <class T>
T sqr(T x) {
  return x * x;
}
template <class T>
T gcd(T a, T b) {
  return (b != 0 ? gcd<T>(b, a % b) : a);
}
template <class T>
T lcm(T a, T b) {
  return (a / gcd<T>(a, b) * b);
}
template <class T>
inline T bigmod(T p, T e, T M) {
  if (e == 0) return 1;
  if (e % 2 == 0) {
    long long int t = bigmod(p, e / 2, M);
    return (T)((t * t) % M);
  }
  return (T)((long long int)bigmod(p, e - 1, M) * (long long int)p) % M;
}
template <class T>
inline T bigexp(T p, T e) {
  if (e == 0) return 1;
  if (e % 2 == 0) {
    long long int t = bigexp(p, e / 2);
    return (T)((t * t));
  }
  return (T)((long long int)bigexp(p, e - 1) * (long long int)p);
}
template <class T>
inline T modinverse(T a, T M) {
  return bigmod(a, M - 2, M);
}
int dx4[] = {1, 0, -1, 0};
int dy4[] = {0, 1, 0, -1};
int dx8[] = {1, 1, 0, -1, -1, -1, 0, 1};
int dy8[] = {0, 1, 1, 1, 0, -1, -1, -1};
int month[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
int n, a[100005], res, l[100005], r[100005];
int main() {
  scanf("%d", &n);
  for (__typeof(n) i = (1); i <= (n); i++) scanf("%d", &a[i]);
  for (__typeof(n) i = (1); i <= (n); i++) l[i] = min(l[i - 1] + 1, a[i]);
  for (__typeof(1) i = (n); i >= (1); i--) r[i] = min(r[i + 1] + 1, a[i]);
  for (__typeof(n) i = (1); i <= (n); i++) {
    a[i] = min(l[i], r[i]);
    res = max(res, a[i]);
  }
  cout << res;
  return 0;
}
