#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[8]={1,0,-1,0,1,1,-1,-1};
const ll dx[8]={0,-1,0,1,1,-1,1,-1};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

int main(){
    ll n,m;cin >> n >>m;
    vl a(m);rep(i,m)cin >> a[i];
    a.pb(0);a.pb(n+1);
    sort(all(a));
    vl div;
    rep(i,m+1){
        ll k=a[i+1]-a[i]-1;
        if(k)div.pb(k);
    }
    if(div.size()==0)cout << 0 <<endl,exit(0);
    //for(auto p:div)cout << p <<endl;
    sort(all(div));
    ll k=div[0];
    ll ans=0;
    for(auto p:div){
        ans+=(p+k-1)/k;
    }
    cout << ans <<endl;
}  
#include <bits/stdc++.h>
using namespace std;

int main() {
  int  n, m;
  cin >> n >> m;
  vector<int> a(m);
  for (int i = 0; i < m; i++) {
    cin >> a[i];
  }
  sort(a.begin(), a.end());
  
  int x = 0;
  int g = n;
  for (int i = 0; i < m; i++) {
    if (a[i] > x + 1) {
      g = min(g, a[i] - x - 1);
    }
    x = a[i];
  }
  x = 0;
  int c = 0;
  for (int i = 0; i < m; i++) {
    if (a[i] > x + 1) {
      c += (a[i] - x - 1) / g + ((a[i] - x -1) % g > 0 ? 1 : 0);
    }
    x = a[i];
  }
  if (x < n) {
    c += (n - x) / g + ((n - x) % g > 0 ? 1 : 0);
  }
  cout << c << endl;
  return 0;
}