#include <bits/stdc++.h>
using namespace std;
long long int mod = 1e9 + 7;
long long int expo(long long int base, long long int exponent,
                   long long int mod) {
  long long int ans = 1;
  while (exponent > 0) {
    if (exponent % 2 == 1) {
      ans *= base;
      ans %= mod;
    }
    base *= base;
    base %= mod;
    exponent /= 2;
  }
  return ans;
}
const long long int M = 3000006;
long long int f[M], inv[M];
void compute() {
  f[0] = 1;
  for (long long int i = 1; i <= M - 1; i++) {
    f[i] = (1LL * i * f[i - 1]) % mod;
  }
  inv[M - 1] = expo(f[M - 1], mod - 2, mod);
  for (long long int i = M - 2; i >= 0; i--) {
    inv[i] = (1LL * (i + 1) * inv[i + 1]) % mod;
  }
}
long long int C(long long int n, long long int r) {
  return (1LL * ((1LL * f[n] * inv[r]) % mod) * inv[n - r]) % mod;
}
long long int h[100005];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int tests = 1;
  while (tests--) {
    long long int i, j, n;
    cin >> n;
    for (long long int i = 1; i <= n; i++) cin >> h[i];
    long long int tmp = 0;
    vector<long long int> res(n + 1);
    for (long long int i = 1; i <= n; i++) {
      tmp = min(tmp, h[i] - i);
      res[i] = tmp + i;
    }
    tmp = 0;
    for (i = n; i >= 1; i--) {
      tmp = min(tmp, h[i] - (n + 1 - i));
      res[i] = min(res[i], tmp + (n + 1 - i));
    }
    long long int ans = 0;
    for (long long int i = 1; i <= n; i++) ans = max(ans, res[i]);
    cout << ans;
  }
  return 0;
}
