#include <iostream>
#include<algorithm>
#include<string>
#include <cmath>
#include <vector>
#include <map>
#include <cstdio>
#include <iomanip>
#include<set>
#include <numeric>
#pragma region Macros
#define int long long
#define double long double
constexpr int MOD = 1000000007;
constexpr double PI = 3.14159265358979323846;
#define rep(i,n) for(int i=0;i<n;i++)
#define rrep(i,n) for(int i=n-1;i>=0;i--)
#define REP(i,n) for(int i=1;i<=n;i++)
#define LAST(x) x[x.size()-1]
#define ALL(x) (x).begin(),(x).end()
#define swap(a,b) (a += b,b = a - b,a -= b)
#define DIVCEIL(a,b) ((a+b-1)/b)
//int CHMAX

int intpow(int a, int n) {
	// a^nのint ver
	rep(i, n - 1)
		a *= a;
	return a;
}
int MODPOW(int a, int n, int mod) {
	// a^n mod
	int ans = 1;
	while (n > 0) {
		if (n & 1)
			ans = ans * a % mod;
		a = a * a % mod;
		n >>= 1;
	}
	return ans;
}
int FACT(int a) {
	if (a == 0)
		return 1;
	else
		return a * FACT(a - 1);
}
int MODFACT(int a, int mod) {
	if (a == 0)
		return 1;
	else
		return (a * FACT(a - 1)) % mod;
}
int nPr(int n, int r) {
	int s = n - r + 1;
	int sum = 1;
	for (int i = s; i <= n; i++)
		sum *= i;
	return sum;
}
int MODnPr(int n, int r, int mod) {
	int s = n - r + 1;
	int sum = 1;
	for (int i = s; i <= n; i++) {
		sum *= i;
		sum = sum % MOD;
	}
	return sum;
}
//int nCr(int n, int r)
int nCr2(int n, int r) {
	return FACT(n) / (FACT(r) * FACT(n - r));
}
int GCD(int a, int b)
{
	if (a < b)
		swap(a, b);
	if (b == 0)
		return a;
	if (a % b == 0)
		return b;
	return GCD(b, a % b);
}
int LCM(int a, int b)
{
	return  a * b / GCD(a, b);
}
int NUMOFDIV(int n) {
	//約数の数（だっけ？）
	int ans = 0;
	REP(i, n) {
		if (n % i == 0)
			ans++;
	}
	return ans;
}
int CEIL1(int n) {
	//1桁目切り上げ
	return (n + 9) / 10 * 10;
}
int DIGIT(int n, int k) {
	//nのk桁目
	rep(i, k - 1)
		n /= 10;
	return n % 10;
}
int DIGITSUM(int n) {
	int sum = 0, dig;
	while (n) {
		dig = n % 10;
		sum += dig;
		n /= 10;
	}
	return sum;
}
int DIVTIME(int n, int k) {
	//nをkで何回割れるか的な
	int div = 0;
	while (n % k == 0) {
		div++;
		n /= k;
	}
	return div;
}
int binary(int n) {
	//10進数→2進数
	int ans = 0;
	for (int i = 0; n > 0; i++) {
		ans += n % 2 * intpow(10, i);
		n /= 2;
	}
	return ans;
}
double LOG(int a, int b) {
	return log(b) / log(a);
}
inline bool BETWEEN(int x, int min, int max) {
	if (min <= x && x <= max)
		return true;
	else
		return false;
}
inline bool between(int x, int min, int max) {
	if (min < x && x < max)
		return true;
	else
		return false;
}
inline bool PRIMEOR(int x) {
	if (x == 1)
		return false;
	if (x == 2)
		return true;
	if (x % 2 == 0)
		return false;
	double sqrtx = sqrt(x);
	for (int i = 3; i <= sqrtx; i += 2) {
		if (x % i == 0)
			return false;
	}
	return true;
}
bool SQRTOR(int n) {
	if (sqrt(n) == (int)sqrt(n))
		return true;
	else
		return false;
}
// 順位付け
using namespace std;
#pragma endregion
signed main() {
	int K, N,x,max=0,maxi=0;
	cin >> K >> N;
	vector<int>A(N);
	rep(i, N)
		cin >> A[i];
	rep(i, N -1) {
		x = A[i + 1] - A[i];
		if (x > max) {
			max = x;
			maxi = i+1;
		}
	}
	x = K - A[N - 1] + A[0];
	if (x > max) {
		max = x;
		maxi = N+1;
	}
	if (maxi != N + 1)
		cout << K - A[maxi] + A[maxi - 1];
	else
		cout << A[N - 1] - A[0];
}

Measured memory_bytes: 1920