/////////// <----------------------  QuickSilver_ ---------------------> ///////////

#include <iostream>
#include <bits/stdc++.h>
#include <sstream>
#include <cmath>
#include <algorithm>
#include <map>
#include <iterator>
#include <iomanip>
#include <queue>
#include <utility>
#include <set>
#include <cstring>

using namespace std;
#define ll long long int
#define c(x) ll x;cin>>x
#define cc(x,y) ll x,y;cin>>x>>y
#define ccc(x,y,z) ll x,y,z; cin>>x>>y>>z
#define db long double
#define fast cin.tie(NULL); cout.tie(NULL); ios_base::sync_with_stdio(false)
#define inf 200000000000000
#define mod 1000000007
#define eps 1e-7
#define PI 3.1415926535897932385
#define pb push_back
#define bitc  __builtin_popcountll
#define mp make_pair
#define ff first
#define ss second
#define endl '\n'
#define all(ar) ar.begin(), ar.end()
#define len(x) (ll)(x).size()
#define fr(i,a,b) for (ll i = (a), _b = (b); i <= _b; i++)
#define rep(i,n) for (ll i = 0, _n = (n); i < _n; i++)
#define repr(i,n) for (ll i = n - 1; i >= 0; i--)
#define frr(i,a,b) for (ll i = (a), _b = (b); i >= _b; i--)
#define foreach(it,ar) for (auto it = ar.begin(); it != ar.end(); it++)
#define fill(ar,val) memset(ar, val, sizeof(ar))
#define print(arr,n) rep(i,n) cout<<arr[i]<<" "; cout<<endl
#define print2(arr,n,m) rep(i,n){ rep(j,m) cout<<arr[i][j]<<" "; cout<<endl; }
#define dyn1(arr,n) ll* arr=new ll[n]; rep(i,n) arr[i]=0;
#define dyn2(arr,n,m) ll** arr=new ll*[(n)]; rep(i,n) arr[i]=new ll[m]; rep(i,n) rep(j,m) arr[i][j]=0
#define carr(arr,n) rep(i,n) cin>>arr[i]
#define carr2(arr,n,m) rep(i,n) rep(j,m) cin>>arr[i][j]
#define debug(a) cout<<" :: "<<#a<<" == "<<a<<endl
#define debug2(a,b) cout<<" :: "<<#a<<" == "<<a<<" :: "<<#b<<" == "<<b<<endl
typedef pair<ll,ll> pii;
typedef vector<pii> vpii;

inline ll maxim(ll a,ll b) {if(a>b) return a; else return b;}
inline ll minim(ll a,ll b) {if(a<b) return a; else return b;}
inline bool equals(double a, double b){ return fabs(a - b) < 1e-9; }
ll gcd(ll a, ll b){ return b==0 ? a : gcd(b, a%b); }


bool prime[3001];
void SieveOfEratosthenes()
{
    memset(prime, true, sizeof(prime));
    prime[1]=false;
    for (int p=2; p*p<=3000; p++)
    {
        if (prime[p] == true)
        {
            for (int i=p*p; i<=3000; i += p)
                prime[i] = false;
        }
    }
}

ll glob_c;

ll ncr[1001][1001];

ll binpow(ll a, ll b)
{
    ll res = 1;
    while (b > 0)
    {
        if (b & 1) res = (res * a);
        a = (a * a);
        b >>= 1;
    }
    return res;
}

ll modpow(ll a, ll b)
{
    ll res = 1;
    while (b > 0)
    {
        if (b & 1) res = (res * a)%mod;
        a = (a * a)%mod;
        b >>= 1;
    }
    return res;
}

ll inv(ll x)
{
  return modpow(x,mod-2);
}

ll arr[1000010]={0};

int lpf[100001] = {};
void precompute()
{
	for (int i = 2; i <= 100000; i++)
		if (!lpf[i])
			for (int j = i; j <= 100000; j += i)
				lpf[j] = i;
}


vector<ll> love;
stack<ll> z;
ll mat[10000][45]={0};
vector<ll> adj[1000010];
bool visited[1000010]={false};

int main()
{
  fast;
  //freopen("input.txt", "r", stdin);
  //freopen("output.txt", "w", stdout);
  cout<<setprecision(10)<<fixed;
  cc(k,n);
  carr(arr,n);
  sort(arr,arr+n);
  ll mino=-1;
  fr(o,1,n-1)
  {
    mino=maxim(mino,arr[o]-arr[o-1]);
  }
  //debug(mino);

  mino=maxim(mino,arr[0]+k-arr[n-1]);
  //debug(mino);
  cout<<k-mino<<endl;
  return 0;
}


Measured memory_bytes: 27904