Limak is a little bear who loves to play. Today he is playing by destroying block towers. He built n towers in a row. The i-th tower is made of hi identical blocks. For clarification see picture for the first sample.

Limak will repeat the following operation till everything is destroyed.

Block is called internal if it has all four neighbors, i.e. it has each side (top, left, down and right) adjacent to other block or to the floor. Otherwise, block is boundary. In one operation Limak destroys all boundary blocks. His paws are very fast and he destroys all those blocks at the same time.

Limak is ready to start. You task is to count how many operations will it take him to destroy all towers.

Input

The first line contains single integer n (1 ≤ n ≤ 105).

The second line contains n space-separated integers h1, h2, ..., hn (1 ≤ hi ≤ 109) — sizes of towers.

Output

Print the number of operations needed to destroy all towers.

Examples

Input

6
2 1 4 6 2 2


Output

3


Input

7
3 3 3 1 3 3 3


Output

2

Note

The picture below shows all three operations for the first sample test. Each time boundary blocks are marked with red color. 

<image> After first operation there are four blocks left and only one remains after second operation. This last block is destroyed in third operation.
import java.io.*;
import java.util.StringTokenizer;

public class _573B {

    public static void main(String[] args) throws Exception {
        Reader.init(System.in);
        BufferedWriter cout = new BufferedWriter(new OutputStreamWriter(System.out));

        int n = Reader.nextInt();
        int[] h = new int[n];
        for (int i = 0; i < n; i++) h[i] = Reader.nextInt();

        int[] l = new int[n], r = new int[n];
        l[0] = 1;
        for (int i = 1; i < n; i++) l[i] = Math.min(h[i], l[i - 1] + 1);
        r[n - 1] = 1;
        for (int i = n - 2; i >= 0; i--) r[i] = Math.min(h[i], r[i + 1] + 1);

        int ans = 0;
        for (int i = 0; i < n; i++) ans = Math.max(ans, Math.min(l[i], r[i]));

        System.out.println(ans);

        cout.close();
    }

    static class Pair<U extends Comparable<U>, V extends Comparable<V>> implements Comparable<Pair<U, V>> {
        final U _1;
        final V _2;

        private Pair(U key, V val) {
            this._1 = key;
            this._2 = val;
        }

        public static <U extends Comparable<U>, V extends Comparable<V>> Pair<U, V> instanceOf(U _1, V _2) {
            return new Pair<U, V>(_1, _2);
        }

        @Override
        public String toString() {
            return _1 + " " + _2;
        }

        @Override
        public int hashCode() {
            int res = 17;
            res = res * 31 + _1.hashCode();
            res = res * 31 + _2.hashCode();
            return res;
        }

        @Override
        public int compareTo(Pair<U, V> that) {
            int res = this._1.compareTo(that._1);
            if (res < 0 || res > 0) return res;
            else return this._2.compareTo(that._2);
        }

        @Override
        public boolean equals(Object obj) {
            if (this == obj) return true;
            if (!(obj instanceof Pair)) return false;
            Pair<?, ?> that = (Pair<?, ?>) obj;
            return _1.equals(that._1) && _2.equals(that._2);
        }
    }

    /**
     * Class for buffered reading int and double values
     */
    static class Reader {
        static BufferedReader reader;
        static StringTokenizer tokenizer;

        /**
         * call this method to initialize reader for InputStream
         */
        static void init(InputStream input) {
            reader = new BufferedReader(new InputStreamReader(input));
            tokenizer = new StringTokenizer("");
        }

        /**
         * get next word
         */
        static String next() throws IOException {
            while (!tokenizer.hasMoreTokens()) {
                // TODO add check for eof if necessary
                tokenizer = new StringTokenizer(reader.readLine());
            }
            return tokenizer.nextToken();
        }

        static int nextInt() throws IOException {
            return Integer.parseInt(next());
        }

        static double nextDouble() throws IOException {
            return Double.parseDouble(next());
        }
    }

    static class ArrayUtil {
        static void swap(int[] a, int i, int j) {
            int tmp = a[i];
            a[i] = a[j];
            a[j] = tmp;
        }

        static void swap(long[] a, int i, int j) {
            long tmp = a[i];
            a[i] = a[j];
            a[j] = tmp;
        }

        static void swap(double[] a, int i, int j) {
            double tmp = a[i];
            a[i] = a[j];
            a[j] = tmp;
        }

        static void swap(char[] a, int i, int j) {
            char tmp = a[i];
            a[i] = a[j];
            a[j] = tmp;
        }

        static void swap(boolean[] a, int i, int j) {
            boolean tmp = a[i];
            a[i] = a[j];
            a[j] = tmp;
        }

        static void reverse(int[] a, int i, int j) {
            for (; i < j; i++, j--)
                swap(a, i, j);
        }

        static void reverse(long[] a, int i, int j) {
            for (; i < j; i++, j--)
                swap(a, i, j);
        }

        static void reverse(double[] a, int i, int j) {
            for (; i < j; i++, j--)
                swap(a, i, j);
        }

        static void reverse(char[] a, int i, int j) {
            for (; i < j; i++, j--)
                swap(a, i, j);
        }

        static void reverse(boolean[] a, int i, int j) {
            for (; i < j; i++, j--)
                swap(a, i, j);
        }

        static long sum(int[] a) {
            int sum = 0;
            for (int i : a)
                sum += i;
            return sum;
        }

        static long sum(long[] a) {
            long sum = 0;
            for (long i : a)
                sum += i;
            return sum;
        }

        static double sum(double[] a) {
            double sum = 0;
            for (double i : a)
                sum += i;
            return sum;
        }

        static int max(int[] a) {
            int max = Integer.MIN_VALUE;
            for (int i : a)
                if (i > max) max = i;
            return max;
        }

        static int min(int[] a) {
            int min = Integer.MAX_VALUE;
            for (int i : a)
                if (i < min) min = i;
            return min;
        }

        static long max(long[] a) {
            long max = Long.MIN_VALUE;
            for (long i : a)
                if (i > max) max = i;
            return max;
        }

        static long min(long[] a) {
            long min = Long.MAX_VALUE;
            for (long i : a)
                if (i < min) min = i;
            return min;
        }
    }
}
