#include <iostream>
#include <cstdlib>
#include <time.h>
#include <algorithm>
#include <map>
#include <queue>
#include <stack>
#include <vector>
#include <array>
#include <string>
#include <iomanip>
#include <cstring>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define mp make_pair
#define eb emplace_back
#define f first
#define s second
#define sz(x) (ll)x.size()
#define lnt(s) (ll)s.length()
#define mt(cnt) cnt.empty()
using namespace std;
using namespace __gnu_pbds;

typedef long long int ll;
typedef tree<ll, null_type, less<ll>, rb_tree_tag, tree_order_statistics_node_update> osSet;

using pll = pair<ll, ll>;
using pl = pair<ll, pll>;
using tll = tuple<ll, ll, ll>;

const ll maxN = 1e5;
const ll INF = 1e17;
const ll MOD = 1e9 + 7;

ll xs[] = {-1, 1, 0, 0, -1, 1, 1, -1};
ll ys[] = {0, 0, -1, 1, -1, 1, -1, 1};

ll segtree[maxN << 2];
ll lazy[maxN << 2];
ll A[maxN];

inline ll left(ll x){
	return x << 1;
}

inline ll right(ll x){
	return (x<<1) + 1;
}

ll func(ll x){
	return (x*(x+1))/2;
}

ll _pow(ll b, ll p){
	if(p == 0) return 1;
	if(p == 1) return b;
	ll exB = b*b;
	if(p&1) return b*_pow(exB, p/2);
	return _pow(exB, p/2);
}

int main(){
	cin.sync_with_stdio(false);
	cin.tie(0);
	ll n, m;
	cin >> n >> m;
	ll ar[m];
	for(ll i=0; i<m; i++)
		cin >> ar[i];
	sort(ar, ar+m);
	vector<ll> v;
	ll low = 0;
	for(ll i=0; i<m; i++){
		if(i == m-1 and ar[i] != n){
			v.emplace_back(n-ar[i]);
		}
		v.emplace_back(ar[i]-low-1);
		low = ar[i];
	}
	if(!(n-m))
		cout << "0";
	else if(!m)
		cout << "1";
	else{
		ll mn = INF, ans = 0;
		for(ll i=0; i<v.size(); i++){
			if(v[i] != 0)
	        	mn = min(mn, v[i]);
        }
		for(ll i=0; i<v.size(); i++)
			ans += v[i]/mn + (v[i]%mn != 0 ? 1 : 0);
		cout << ans;
	}
	return 0;
}
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
// --------------------------------------------------------
template <class T>
bool chmax(T& a, const T b) {
    if (a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <class T>
bool chmin(T& a, const T b) {
    if (b < a) {
        a = b;
        return 1;
    }
    return 0;
}
#define FOR(i, l, r) for (int i = (l); i < (r); ++i)
#define RFOR(i, l, r) for (int i = (r)-1; (l) <= i; --i)
#define REP(i, n) FOR(i, 0, n)
#define RREP(i, n) RFOR(i, 0, n)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define SORT(c) sort(ALL(c))
#define RSORT(c) sort(RALL(c))
#define MIN(c) *min_element(ALL(c))
#define MAX(c) *max_element(ALL(c))
#define SUM(c) accumulate(ALL(c), 0)
#define SUMLL(c) accumulate(ALL(c), 0LL)
#define SZ(c) ((int)(c).size())
#define CIN(c) cin >> (c)
#define COUT(c) cout << (c) << '\n'
#define debug(x) cerr << #x << " = " << (x) << '\n';
using P = pair<ll, ll>;
using VP = vector<P>;
using VVP = vector<VP>;
using VS = vector<string>;
using VI = vector<int>;
using VVI = vector<VI>;
using VLL = vector<ll>;
using VVLL = vector<VLL>;
using VB = vector<bool>;
using VVB = vector<VB>;
using VD = vector<double>;
using VVD = vector<VD>;
static const double EPS = 1e-10;
static const double PI = acos(-1.0);
static const ll MOD = 1000000007;
// static const ll MOD = 998244353;
static const int INF = (1 << 30) - 1; // 1073741824 - 1
// static const ll INF = (1LL << 60) - 1;  // 4611686018427387904 - 1

const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    ll N, M;
    cin >> N >> M;
    if (M == 0) {
        COUT(1);
        return 0;
    }

    VLL A(M);
    REP(i, M) {
        CIN(A[i]);
    }
    SORT(A);

    ll minVal = INF;
    ll bef = 1;
    VI between;
    A.push_back(N + 1);
    // A.push_back(N);
    REP(i, M + 1) {
        ll p = A[i] - bef;
        bef = A[i] + 1;
        if (p <= 0) continue;
        chmin(minVal, p);
        between.push_back(p);
    }

    ll ans = 0;
    for (auto v : between) {
        ans += ceil(v / (double)minVal);
    }
    COUT(ans);
    return 0;
}