import java.util.*;
import java.io.*;
import java.math.*;
 
public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int k = sc.nextInt();
        int n = sc.nextInt();
        ArrayList<Integer> list = new ArrayList<>();
        for(int i = 0; i < n; i++){
            list.add(sc.nextInt());
        }
        Collections.sort(list);
        int ans = list.get(n-1)-list.get(0);
        for(int i = 1; i < n; i++){
            ans = Math.min(ans, k-(list.get(i) - list.get(i-1)));
        }
        System.out.println(ans);
    }

}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz