import java.io.*;
import java.util.*;

public class Main {

	public static void main(String[] args) throws Exception {
		
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		
		StringTokenizer st = new StringTokenizer(in.readLine());
		
		int K = Integer.parseInt(st.nextToken());
		int N = Integer.parseInt(st.nextToken());
		
		int[] nums = new int[2*N];
		
		st = new StringTokenizer(in.readLine());
		for(int i = 0; i < N; i++) {
			nums[i] = Integer.parseInt(st.nextToken());
			nums[i+N] = nums[i] + K;
		}
		
		int min = Integer.MAX_VALUE;
		for(int i = 0; i + N-1 < 2*N; i++) {
			min = Math.min(min, nums[i+N-1] - nums[i]);
		}
		
		System.out.println(min);
	}
} import java.io.*; io.* java import java.util.*; util.* java public class Main {

	public static void main(String[] args) throws Exception {
		
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		
		StringTokenizer st = new StringTokenizer(in.readLine());
		
		int K = Integer.parseInt(st.nextToken());
		int N = Integer.parseInt(st.nextToken());
		
		int[] nums = new int[2*N];
		
		st = new StringTokenizer(in.readLine());
		for(int i = 0; i < N; i++) {
			nums[i] = Integer.parseInt(st.nextToken());
			nums[i+N] = nums[i] + K;
		}
		
		int min = Integer.MAX_VALUE;
		for(int i = 0; i + N-1 < 2*N; i++) {
			min = Math.min(min, nums[i+N-1] - nums[i]);
		}
		
		System.out.println(min);
	}
} Main public static void main(String[] args) throws Exception {
		
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		
		StringTokenizer st = new StringTokenizer(in.readLine());
		
		int K = Integer.parseInt(st.nextToken());
		int N = Integer.parseInt(st.nextToken());
		
		int[] nums = new int[2*N];
		
		st = new StringTokenizer(in.readLine());
		for(int i = 0; i < N; i++) {
			nums[i] = Integer.parseInt(st.nextToken());
			nums[i+N] = nums[i] + K;
		}
		
		int min = Integer.MAX_VALUE;
		for(int i = 0; i + N-1 < 2*N; i++) {
			min = Math.min(min, nums[i+N-1] - nums[i]);
		}
		
		System.out.println(min);
	} main {
		
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		
		StringTokenizer st = new StringTokenizer(in.readLine());
		
		int K = Integer.parseInt(st.nextToken());
		int N = Integer.parseInt(st.nextToken());
		
		int[] nums = new int[2*N];
		
		st = new StringTokenizer(in.readLine());
		for(int i = 0; i < N; i++) {
			nums[i] = Integer.parseInt(st.nextToken());
			nums[i+N] = nums[i] + K;
		}
		
		int min = Integer.MAX_VALUE;
		for(int i = 0; i + N-1 < 2*N; i++) {
			min = Math.min(min, nums[i+N-1] - nums[i]);
		}
		
		System.out.println(min);
	} BufferedReader in = new BufferedReader(new InputStreamReader(System.in)); in new BufferedReader(new InputStreamReader(System.in)) StringTokenizer st = new StringTokenizer(in.readLine()); st new StringTokenizer(in.readLine()) int K = Integer.parseInt(st.nextToken()); K Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st int N = Integer.parseInt(st.nextToken()); N Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st int[] nums = new int[2*N]; nums new int[2*N] 2*N 2 N st = new StringTokenizer(in.readLine()) st new StringTokenizer(in.readLine()) for(int i = 0; i < N; i++) {
			nums[i] = Integer.parseInt(st.nextToken());
			nums[i+N] = nums[i] + K;
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			nums[i] = Integer.parseInt(st.nextToken());
			nums[i+N] = nums[i] + K;
		} {
			nums[i] = Integer.parseInt(st.nextToken());
			nums[i+N] = nums[i] + K;
		} nums[i] = Integer.parseInt(st.nextToken()) nums[i] nums i Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st nums[i+N] = nums[i] + K nums[i+N] nums i+N i N nums[i] + K nums[i] nums i K int min = Integer.MAX_VALUE; min Integer.MAX_VALUE Integer Integer.MAX_VALUE for(int i = 0; i + N-1 < 2*N; i++) {
			min = Math.min(min, nums[i+N-1] - nums[i]);
		} int i = 0; int i = 0; i 0 i + N-1 < 2*N i + N-1 i + N i N 1 2*N 2 N i++ i++ i {
			min = Math.min(min, nums[i+N-1] - nums[i]);
		} {
			min = Math.min(min, nums[i+N-1] - nums[i]);
		} min = Math.min(min, nums[i+N-1] - nums[i]) min Math.min(min, nums[i+N-1] - nums[i]) Math.min Math min nums[i+N-1] - nums[i] nums[i+N-1] nums i+N-1 i+N i N 1 nums[i] nums i System.out.println(min) System.out.println System.out System System.out min String[] args args public class Main {

	public static void main(String[] args) throws Exception {
		
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		
		StringTokenizer st = new StringTokenizer(in.readLine());
		
		int K = Integer.parseInt(st.nextToken());
		int N = Integer.parseInt(st.nextToken());
		
		int[] nums = new int[2*N];
		
		st = new StringTokenizer(in.readLine());
		for(int i = 0; i < N; i++) {
			nums[i] = Integer.parseInt(st.nextToken());
			nums[i+N] = nums[i] + K;
		}
		
		int min = Integer.MAX_VALUE;
		for(int i = 0; i + N-1 < 2*N; i++) {
			min = Math.min(min, nums[i+N-1] - nums[i]);
		}
		
		System.out.println(min);
	}
} public class Main {

	public static void main(String[] args) throws Exception {
		
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		
		StringTokenizer st = new StringTokenizer(in.readLine());
		
		int K = Integer.parseInt(st.nextToken());
		int N = Integer.parseInt(st.nextToken());
		
		int[] nums = new int[2*N];
		
		st = new StringTokenizer(in.readLine());
		for(int i = 0; i < N; i++) {
			nums[i] = Integer.parseInt(st.nextToken());
			nums[i+N] = nums[i] + K;
		}
		
		int min = Integer.MAX_VALUE;
		for(int i = 0; i + N-1 < 2*N; i++) {
			min = Math.min(min, nums[i+N-1] - nums[i]);
		}
		
		System.out.println(min);
	}
} Main
import java.io.*;
import java.util.*;

public class Main {

	public static void main(String[] args) throws Exception {
		
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		
		StringTokenizer st = new StringTokenizer(in.readLine());
		
		int K = Integer.parseInt(st.nextToken());
		int N = Integer.parseInt(st.nextToken());
		
		int[] nums = new int[2*N];
		
		st = new StringTokenizer(in.readLine());
		for(int i = 0; i < N; i++) {
			nums[i] = Integer.parseInt(st.nextToken());
			nums[i+N] = nums[i] + K;
		}
		
		int min = Integer.MAX_VALUE;
		for(int i = 0; i + N-1 < 2*N; i++) {
			min = Math.min(min, nums[i+N-1] - nums[i]);
		}
		
		System.out.println(min);
	}
}
