//     Author: Rodney Shaghoulian
//     Github: github.com/RodneyShag
// HackerRank: hackerrank.com/RodneyShag

import java.util.Scanner;

// Create a boolean array for the cities. "true" indicates there's a station in the city.
//
// There are 3 cases:
//  - You are at left of 1st station - in this case, you are forced to walk right
//  - You are at right of last station - in this case, you are forced to walk left
//  - You are between 2 stations - you walk to nearest station

// Time complexity: O(n)
public class Solution {
    public static void main(String[] args) {
        /* Read input - Create chain of cities */
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        int m = scan.nextInt();
        boolean[] cities = new boolean[n];
        while (m-- > 0) {
            int station = scan.nextInt();
            cities[station] = true;
        }
        scan.close();
        
        System.out.println(findMin(cities));
    }
    
    static int findMin(boolean[] cities) {
        int lastStation = -1;
        int distance    =  0;
        int max         =  0;
        for (int i = 0; i < cities.length; i++) {
            if (cities[i]) {
                if (lastStation == -1) { // accounts for being left of 1st station
                    max = i;
                } else { // accounts for being in between 2 stations
                    distance = i - lastStation;
                    max = Math.max(max, distance / 2);
                }
                lastStation = i;
            }
        }
        /* Account for being right of last station */
        distance = cities.length - 1 - lastStation;
        max = Math.max(max, distance);
        return max;
    }
}