content: import java.io.*;
  import java.math.*;
  import java.security.*;
  import java.text.*;
  import java.util.*;
  import java.util.concurrent.*;
  import java.util.regex.*;
  
  public class Solution {
  
    // Complete the flatlandSpaceStations function below.
    static int flatlandSpaceStations(int n, int[] c) {
      Arrays.sort(c);
      int max = Integer.MIN_VALUE;
      for (int i = 1; i < c.length; i++) {
        max = Math.max(max, (c[i] - c[i - 1]) / 2);
      }
      max = Math.max(max, c[0]);
      max = Math.max(max, (n - 1 - c[c.length - 1]));
      return max;
    }
  
    private static final Scanner scanner = new Scanner(System.in);
  
    public static void main(String[] args) throws IOException {
      BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));
  
      String[] nm = scanner.nextLine().split(" ");
  
      int n = Integer.parseInt(nm[0]);
  
      int m = Integer.parseInt(nm[1]);
  
      int[] c = new int[m];
  
      String[] cItems = scanner.nextLine().split(" ");
      scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");
  
      for (int i = 0; i < m; i++) {
        int cItem = Integer.parseInt(cItems[i]);
        c[i] = cItem;
      }
  
      int result = flatlandSpaceStations(n, c);
  
      bufferedWriter.write(String.valueOf(result));
      bufferedWriter.newLine();
  
      bufferedWriter.close();
  
      scanner.close();
    }
  }
  