using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

class Program
{
    static void Main()
    {
 int n = int.Parse(Console.ReadLine());
            int[] locations = new int[n];

            for (int i = 0; i < locations.Length; i++)
            {
                locations[i] = 0;
            }

            int[] safeHouse = Console.ReadLine().Split().Select(int.Parse).ToArray();

            for (int i = 0; i < locations.Length; i++)
            {
                if (safeHouse.Contains(i))
                {
                    locations[i] = 1;
                }
            }

            int maxDistance = 0;
            int currentLenght = 0;

            for (int i = 0; i < locations.Length; i++)
            {
                if (locations[i] == 0)
                {
                    currentLenght++;
                }
                else
                {
                    if (maxDistance < currentLenght)
                    {
                        maxDistance = currentLenght;
                    }
                    currentLenght = 0;
                }

            }

            if (safeHouse.Length > 1)
            {
                maxDistance = (int)Math.Ceiling(maxDistance / 2.0);
            }
            else if (locations[0] == 1 && locations[locations.Length - 1] == 1)
            {
                maxDistance = locations.Length - 3;
            }
            Console.WriteLine(maxDistance);
    }
}