//#define NDEBUG

#pragma GCC optimize ("O3") // 最適化レベルの変更 O0〜O3 などを指定
#pragma GCC optimize("-O3,inline,omit-frame-pointer,unroll-loops")
#pragma warning(1:4456) // 多重forループ内での変数の２重定義を警告にする

#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES

#include <iostream>

#include <array>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <algorithm>
#include <numeric>
#include <iomanip>
#include <string>
#include <sstream>
#include <fstream>
#include <cmath>
#include <bitset>

#include <memory>
#include <cassert>

#include <functional>
#include <chrono>

using namespace std;

namespace ValLib {

	typedef unsigned int uint;
	typedef long long ll;
	typedef unsigned long long ull;

	constexpr ull V_MOD = 1000000007ull;
	constexpr int V_INT_MAX = 2147483647;
	constexpr ll V_LL_MAX = 9223372036854775807ll;
	constexpr ull V_ULL_MAX = 18446744073709551615ull;

	template<typename Key, typename Value>
	using umap = std::unordered_map<Key, Value>;
	template<typename T>
	using uset = std::unordered_set<T>;

	template<typename T>void fill(vector<T>&vec,const T&value){std::fill(vec.begin(),vec.end(),value);}
	template<typename T>void fill(vector<vector<T>>&vec,const T&value){for(vector<T>&t:vec)std::fill(t.begin(),t.end(),value);}
	template<typename T>void resize(vector<T>&vec,int size1){vec.resize(size1);}
	template<typename T>void resize(vector<T>&vec,int size1,const T& value){vec.resize(size1,value);}
	template<typename T>void resize(vector<vector<T>>&vec,int size1,int size2){vec.resize(size1);for(vector<T>&t:vec)t.resize(size2);}
	template<typename T>void resize(vector<vector<T>>&vec,int size1,int size2,const T& value){vec.resize(size1);for(vector<T>&t:vec)t.resize(size2,value);}
	template<typename T>void assign(vector<T>&vec,int size1,const T& value){vec.assign(size1,value);}
	template<typename T>void assign(vector<vector<T>>&vec,int size1,int size2,const T& value){vec.resize(size1);for(vector<T>&t:vec)t.assign(size2,value);}
	template<typename T>const typename vector<T>::const_iterator max_element(const vector<T>&vec){assert(!vec.empty());return std::max_element(vec.begin(),vec.end());}
	template<typename T,typename _Pr>const typename vector<T>::const_iterator max_element(const vector<T>&vec,_Pr lessThan){assert(!vec.empty());return std::max_element(vec.begin(),vec.end(),lessThan);}
	template<typename T>typename vector<T>::iterator min_element(vector<T>&vec){assert(!vec.empty());return std::min_element(vec.begin(),vec.end());}
	template<typename T>const typename vector<T>::const_iterator min_element(const vector<T>&vec){assert(!vec.empty());return std::min_element(vec.begin(),vec.end());}
	template<typename T,typename _Pr>const typename vector<T>::const_iterator min_element(const vector<T>&vec,_Pr lessThan){assert(!vec.empty());return std::min_element(vec.begin(),vec.end(),lessThan);}
	int accumulate(const vector<int>&vec){return std::accumulate(vec.begin(),vec.end(),0);}
	template<typename _Pr>int accumulate(const vector<int>&vec,_Pr func){return std::accumulate(vec.begin(),vec.end(),0,func);}
	double accumulate(const vector<double>&vec){return std::accumulate(vec.begin(),vec.end(),0.0);}
	template<typename _Pr>double accumulate(const vector<double>&vec,_Pr func){return std::accumulate(vec.begin(),vec.end(),0.0,func);}
	float accumulate(const vector<float>&vec){return std::accumulate(vec.begin(),vec.end(),0.f);}
	template<typename _Pr>float accumulate(const vector<float>&vec,_Pr func){return std::accumulate(vec.begin(),vec.end(),0.f,func);}
	template<typename T,typename _Pr>bool all_of(const vector<T>&vec,_Pr pred){return std::all_of(vec.begin(),vec.end(),pred);	}
	template<typename T,typename _Pr>bool any_of(const vector<T>&vec,_Pr pred){return std::any_of(vec.begin(),vec.end(),pred);}
	template<typename T,typename _Pr>bool none_of(const vector<T>&vec,_Pr pred){return std::none_of(vec.begin(),vec.end(),pred);}
	template<typename T>const typename vector<T>::const_iterator find(const vector<T>&vec,const T& val){return std::find(vec.begin(),vec.end(),val);}
	template<typename T,typename _Pr>const typename vector<T>::const_iterator find_if(const vector<T>&vec,_Pr pred){return std::find_if(vec.begin(),vec.end(),pred);}
	template<typename T>bool contains(const vector<T>&vec,const T& val){return std::find(vec.begin(),vec.end(),val) != vec.end();}
	template<typename T,typename _Pr>bool contains_if(const vector<T>&vec,_Pr pred){return std::find_if(vec.begin(),vec.end(),pred)!=vec.end();}
	template<typename T,typename _Pr>typename iterator_traits<const typename vector<T>::const_iterator>::difference_type count_if(const vector<T>&vec,_Pr pred){return std::count_if(vec.begin(),vec.end(),pred);}
	template<typename T>void sort(vector<T>&vec){std::sort(vec.begin(),vec.end());}
	template<typename T,typename _Pr>void sort(vector<T>&vec,_Pr pred){std::sort(vec.begin(),vec.end(),pred);}
	template<typename T>void stable_sort(vector<T>&vec){std::stable_sort(vec.begin(),vec.end());}
	template<typename T,typename _Pr>void stable_sort(vector<T>&vec,_Pr pred){std::stable_sort(vec.begin(),vec.end(),pred);}

	template<typename T,size_t N>void fill(array<T,N>&ary,const T& value){std::fill(ary.begin(),ary.end(),value);}
	template<typename T,size_t N,size_t M>void fill(array<array<T,M>,N>&ary,const T& value){ for(array<T,M>& t : ary) std::fill(t.begin(),t.end(),value);}
	template<typename T,size_t N,typename _Pr>const typename array<T,N>::const_iterator max_element(const array<T,N>&ary){assert(!ary.empty());return std::max_element(ary.begin(),ary.end());}
	template<typename T,size_t N,typename _Pr>const typename vector<T>::const_iterator max_element(const array<T,N>&ary){assert(!ary.empty());return std::max_element(ary.begin(),ary.end());}
	template<typename T,size_t N,typename _Pr>const typename array<T,N>::const_iterator max_element(const array<T,N>&ary,_Pr lessThan){assert(!ary.empty());return std::max_element(ary.begin(),ary.end(),lessThan);}
	template<typename T,size_t N,typename _Pr>const typename array<T,N>::const_iterator min_element(const array<T,N>&ary){assert(!ary.empty());return std::min_element(ary.begin(),ary.end());}
	template<typename T,size_t N,typename _Pr>const typename array<T,N>::const_iterator min_element(const array<T,N>&ary,_Pr lessThan){assert(!ary.empty());return std::min_element(ary.begin(),ary.end(),lessThan);}
	template<size_t N>int accumulate(const array<int,N>&ary){return std::accumulate(ary.begin(),ary.end(),0);}
	template<size_t N,typename _Pr>int accumulate(const array<int,N>&ary,_Pr func){return std::accumulate(ary.begin(),ary.end(),0,func);}
	template<size_t N>double accumulate(const array<double,N>&ary){return std::accumulate(ary.begin(),ary.end(),0.0);}
	template<size_t N,typename _Pr>double accumulate(const array<double,N>&ary,_Pr func){return std::accumulate(ary.begin(),ary.end(),0.0,func);}
	template<size_t N> float accumulate(const array<float,N>&ary){return std::accumulate(ary.begin(),ary.end(),0.f);}
	template<size_t N,typename _Pr>float accumulate(const array<float,N>&ary,_Pr func){return std::accumulate(ary.begin(),ary.end(),0.f,func);}
	template<typename T,size_t N,typename _Pr>bool all_of(const array<T,N>&ary,_Pr pred){return std::all_of(ary.begin(),ary.end(),pred);}
	template<typename T,size_t N,typename _Pr>bool any_of(const array<T,N>&ary,_Pr pred){return std::any_of(ary.begin(),ary.end(),pred);}
	template<typename T,size_t N,typename _Pr>bool none_of(const array<T,N>&ary,_Pr pred){return std::none_of(ary.begin(),ary.end(),pred);}
	template<typename T,size_t N>const typename array<T,N>::const_iterator find(const array<T,N>&ary,const T& val){return std::find(ary.begin(),ary.end(),val);}
	template<typename T,size_t N,typename _Pr>const typename array<T,N>::const_iterator find_if(const array<T,N>&ary,_Pr pred){return std::find_if(ary.begin(),ary.end(),pred);}
	template<typename T,size_t N>bool contains(const array<T,N>&ary,const T& val){return std::find(ary.begin(),ary.end(),val)!=ary.end();}
	template<typename T,size_t N,typename _Pr>bool contains_if(const array<T,N>&ary,_Pr pred){return std::find_if(ary.begin(),ary.end(),pred) != ary.end();}
	template<typename T,size_t N,typename _Pr>typename iterator_traits<const typename array<T,N>::const_iterator>::difference_type count_if(const array<T,N>&ary,_Pr pred){return std::count_if(ary.begin(),ary.end(),pred);}
	template<typename T,size_t N>void sort(array<T,N>&ary){std::sort(ary.begin(),ary.end());}
	template<typename T,size_t N,typename _Pr>void sort(array<T,N>&ary,_Pr pred){std::sort(ary.begin(),ary.end(),pred);}
	template<typename T,size_t N>void stable_sort(array<T,N>&ary){std::stable_sort(ary.begin(),ary.end());}
	template<typename T,size_t N,typename _Pr>void stable_sort(array<T,N>&ary,_Pr pred){std::stable_sort(ary.begin(),ary.end(),pred);}

	template<typename Key, typename Value>
	bool containsKey(const umap<Key, Value>& m, const Key& key) {
		return m.find(key) != m.end();
	}
	template<typename Key, typename Value>
	bool containsValue(const umap<Key, Value>& m, const Value& val) {
		for (auto it = m.begin(); it != m.end(); ++it)
			if (it->second == val)
				return true;
		return false;
	}
	template<typename T>
	const typename uset<T>::const_iterator find(const uset<T>& s, const T& key) {
		return s.find(key);
	}
	template<typename T>
	bool contains(const uset<T>& s, const T& key) {
		return s.find(key) != s.end();
	}

	constexpr int DX4[] = {0, -1, 0, 1};
	constexpr int DY4[] = {-1, 0, 1, 0};
	constexpr int DX8[] = {0, -1, -1, -1, 0, 1, 1, 1};
	constexpr int DY8[] = {-1, -1, 0, 1, 1, 1, 0, -1};

	constexpr int DX5[] = {DX4[0], DX4[1], DX4[2], DX4[3], 0};
	constexpr int DY5[] = {DY4[0], DY4[1], DY4[2], DY4[3], 0};
	constexpr int DX9[] = {DX8[0], DX8[1], DX8[2], DX8[3], DX8[4], DX8[5], DX8[6], DX8[7], 0};
	constexpr int DY9[] = {DY8[0], DY8[1], DY8[2], DY8[3], DY8[4], DY8[5], DY8[6], DY8[7], 0};

	class Point {
	public:

		constexpr inline Point() : Point(-1, -1) {}
		constexpr inline Point(int x, int y) : x(x), y(y)	{}

		static Point getManhattanDist(const Point& p1, const Point& p2) {
			return move(Point(abs(p1.x - p2.x), abs(p1.y - p2.y))); 
		}

		void setPos(const Point& pos) { x = pos.x; y = pos.y; }
		void setPos(int x, int y) { this->x = x; this->y = y; }

		Point operator+ (int val) const { return move(Point(x + val, y + val)); }
		Point operator- (int val) const { return move(Point(x - val, y - val)); }
		Point operator* (int val) const { return move(Point(x * val, y * val)); }
		Point operator/ (int val) const { return move(Point(x / val, y / val)); }
		Point operator+ (const Point &p) const { return move(Point(x + p.x, y + p.y)); }
		Point operator- (const Point &p) const { return move(Point(x - p.x, y - p.y)); }
		Point operator* (const Point &p) const { return move(Point(x * p.x, y * p.y)); }
		Point operator/ (const Point &p) const { return move(Point(x / p.x, y / p.y)); }
		void operator+= (int val) { x += val; y += val; }
		void operator-= (int val) { x -= val; y -= val; }
		void operator*= (int val) { x *= val; y *= val; }
		void operator/= (int val) { x /= val; y /= val; }
		void operator+= (const Point &p) { x += p.x; y += p.y; }
		void operator-= (const Point &p) { x -= p.x; y -= p.y; }
		void operator*= (const Point &p) { x *= p.x; y *= p.y; }
		void operator/= (const Point &p) { x /= p.x; y /= p.y; }
		bool operator== (const Point &p) const { return x == p.x && y == p.y; }
		bool operator!= (const Point &p) const { return x != p.x || y != p.y; }
		//bool operator<(const Point &p) const {return x * x + y * y < p.x * p.x + p.y * p.y;}
		const Point& getPos() const { return *this; }

		string to_string() const { return "(" + std::to_string(x) + ", " + std::to_string(y) + ")"; }

		int x, y;
	};

	constexpr Point DP4[] = {Point(DX4[0], DY4[0]), Point(DX4[1], DY4[1]), Point(DX4[2], DY4[2]), Point(DX4[3], DY4[3])};
	constexpr Point DP5[] = {Point(DX5[0], DY5[0]), Point(DX5[1], DY5[1]), Point(DX5[2], DY5[2]), Point(DX5[3], DY5[3]), Point(DX5[4], DY5[4])};
	constexpr Point DP8[] = {Point(DX8[0], DY8[0]), Point(DX8[1], DY8[1]), Point(DX8[2], DY8[2]), Point(DX8[3], DY8[3]), Point(DX8[4], DY8[4]), Point(DX8[5], DY8[5]), Point(DX8[6], DY8[6]), Point(DX8[7], DY8[7])};
	constexpr Point DP9[] = {Point(DX9[0], DY9[0]), Point(DX9[1], DY9[1]), Point(DX9[2], DY9[2]), Point(DX9[3], DY9[3]), Point(DX9[4], DY9[4]), Point(DX9[5], DY9[5]), Point(DX9[6], DY9[6]), Point(DX9[7], DY9[7])};

	// 文字列の先頭の0を削除する
	string cutZeroLeft(const string& str) {
		for (int i = 0; i < (int)str.length(); ++i) {
			if (str[i] != '0') {
				return str.substr(i);
			}
		}
		return "";
	}

	// 0以上の整数を0埋めで表記する
	string fillZero(ull number, int digit) {
		int zeroNum = max(0, (int)(digit - to_string(number).length()));
		return move(move(string(zeroNum, '0')) + move(to_string(number)));
	}

	// 0以上の10進数を2進数に変換する
	string toBinString(ull x) {
		return move(cutZeroLeft(bitset<20>(x).to_string()));
	}

	// 0以上の10進数を8進数に変換する
	string toOctString(ull x) {
		stringstream ss;
		ss << std::oct << x;
		return move(ss.str());
	}

	// 0以上の整数を16進数に変換する(a～fは小文字)
	string toHexString(ull x) {
		stringstream ss;
		ss << std::hex << x;
		return move(ss.str());
	}

	// 0以上の整数を任意の基数に変換する(最大36進数)(a～zは小文字)
	string toBaseString(ull x, int base) {
		const string table = "0123456789abcdefghijklmnopqrstuvwxyz";

		assert(base >= 2 && base <= 36);

		string result = "";
		ull y = x;
		do {
			ull div = y / (ull)base;
			int mod = y % (ull)base;

			result = table[mod] + result;
			y = div;
		}
		while (y != 0ull);

		return move(result);
	}

	// 任意の基数の0以上の整数を10進数に変換する
	// base: 変換元の基数
	ull toDecimal(string numberString, int base) {
		assert(numberString.length() < 20);
		assert(!numberString.empty());
		return stoull(numberString, nullptr, base);
	}

	// double型の値を任意の少数桁数で表示する
	// 指定の桁数まで0で埋める。指数表記にはならない
	// precision=0を指定したときは小数点は表示しない
	// 表示桁数＋１桁目を四捨五入する
	string sprintDoublePlane(double value, int precision) {
		assert(precision >= 0);
		stringstream ss;
		ss << fixed << setprecision(precision) << value;
		return move(ss.str());
	}

	template<typename A>void in(A&a){cin>>a;cin.ignore();}
	template<typename A,typename B>void in(A&a,B&b){cin>>a>>b;cin.ignore();}
	template<typename A,typename B,typename C>void in(A&a,B&b,C&c){cin>>a>>b>>c;cin.ignore();}
	template<typename A,typename B,typename C,typename D>void in(A&a,B&b,C&c,D&d){cin>>a>>b>>c>>d;cin.ignore();}
	template<typename A,typename B,typename C,typename D,typename E>void in(A&a,B&b,C&c,D&d,E&e){cin>>a>>b>>c>>d>>e;cin.ignore();}
	template<typename A,typename B,typename C,typename D,typename E,typename F>void in(A&a,B&b,C&c,D&d,E&e,F&f){cin>>a>>b>>c>>d>>e>>f;cin.ignore();}
	template<typename A,typename B,typename C,typename D,typename E,typename F,typename G>void in(A&a,B&b,C&c,D&d,E&e,F&f,G&g){cin>>a>>b>>c>>d>>e>>f>>g;cin.ignore();}

	template<typename A>
	void inr(vector<A>& a, int size) {
		resize(a, size);
		for (int i = 0; i < size; ++i) {
			cin >> a[i]; cin.ignore();
		}
	}
	template<typename A, typename B>
	void inr(vector<A>& a, vector<B>& b, int size) {
		resize(a, size); resize(b, size);
		for (int i = 0; i < size; ++i) {
			cin >> a[i] >> b[i]; cin.ignore();
		}
	}
	template<typename A, typename B, typename C>
	void inr(vector<A>& a, vector<B>& b, vector<C>& c, int size) {
		resize(a, size); resize(b, size); resize(c, size);
		for (int i = 0; i < size; ++i) {
			cin >> a[i] >> b[i] >> c[i]; cin.ignore();
		}
	}
	template<typename A, typename B, typename C, typename D>
	void inr(vector<A>& a, vector<B>& b, vector<C>& c, vector<D>& d, int size) {
		resize(a, size); resize(b, size); resize(c, size); resize(d, size);
		for (int i = 0; i < size; ++i) {
			cin >> a[i] >> b[i] >> c[i] >> d[i]; cin.ignore();
		}
	}
	template<typename A, typename B, typename C, typename D, typename E>
	void inr(vector<A>& a, vector<B>& b, vector<C>& c, vector<D>& d, vector<E>& e, int size) {
		resize(a, size); resize(b, size); resize(c, size); resize(d, size); resize(e, size);
		for (int i = 0; i < size; ++i) {
			cin >> a[i] >> b[i] >> c[i] >> d[i] >> e[i]; cin.ignore();
		}
	}

	template<typename A>
	void inr(vector<vector<A>>& a, int h, int wa) {
		resize(a, h, wa);
		for (int i = 0; i < h; ++i) {
			for (int j = 0; j < wa; ++j) {
				cin >> a[i][j];
			}
			cin.ignore();
		}
	}
	template<typename A,typename B>
	void inr(vector<vector<A>>& a, vector<vector<B>>& b, int h, int wa, int wb) {
		resize(a, h, wa); resize(b, h, wb);
		for (int i = 0; i < h; ++i) {
			for (int j = 0; j < wa; ++j) {
				cin >> a[i][j];
			}
			for (int j = 0; j < wb; ++j) {
				cin >> b[i][j];
			}
			cin.ignore();
		}
	}
	template<typename A,typename B,typename C>
	void inr(vector<vector<A>>& a, vector<vector<B>>& b, vector<vector<C>>& c, int h, int wa, int wb, int wc) {
		resize(a, h, wa); resize(b, h, wb); resize(c, h, wc);
		for (int i = 0; i < h; ++i) {
			for (int j = 0; j < wa; ++j) {
				cin >> a[i][j];
			}
			for (int j = 0; j < wb; ++j) {
				cin >> b[i][j];
			}
			for (int j = 0; j < wc; ++j) {
				cin >> c[i][j];
			}
			cin.ignore();
		}
	}

	template<typename T>
	void out(const T& val) { cout << val << endl; }
}

using namespace ValLib;

int K, N;
vector<int> a;

int main() {

	in(K, N);
	inr(a, N);

	int sum = a[0] + (K - a[N - 1]);
	int maxVal = sum;
	for (int i = 1; i < N; ++i) {
		int dist = a[i] - a[i - 1];
		sum += dist;
		maxVal = max(dist, maxVal);
	}
	out(sum - maxVal);

}


Measured memory_bytes: 1024