#include <bits/stdc++.h>
#define pb push_back
#define fr first
#define sc second
#define bg begin()
#define ed end()
#define all(x) x.bg, x.ed
#define sz(x) int(x.size())
#define vi vector<int> 
#define vl vector<long long>
#define vvl vector<vector<long long> >  
#define vvi vector<vector<int> >
typedef long long ll;
typedef unsigned long long ull;
typedef unsigned int uint;	
using namespace std;
void solve(){
	int n, m, inp;
	cin>>n>>m;
	vi A;
	A.pb(0);
	A.pb(n + 1);
	for(int i = 0; i < m; ++i){
		cin>>inp;
		A.pb(inp);
	}
	sort(all(A));
	int mn = 1e9, ans = 0;
	for(int i = 1; i <= m + 1; ++i){
		if(A[i] - A[i-1] > 1) mn = min(mn, A[i] - A[i-1] - 1);
	}
	for(int i = 1; i <= m + 1; ++i){
		if(A[i] - A[i-1] > 1) ans += (A[i] - A[i-1] - 1)/mn + ((A[i] - A[i-1] - 1)%mn != 0);
	}
	cout<<ans;
}
int main(){
	ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
	solve();
}
#include <bits/stdc++.h>
#define ll long long
#define pb push_back
using namespace std;
void fun(vector<ll>v,ll n,ll m)
{
    v.pb(n+1);
    ll prev=0;
    vector<ll>temp;
    for(ll val:v)
    {
        if(val-prev-1>0)
        {
            temp.pb(val-prev-1);
        }
        prev=val;
    }
    sort(temp.begin(),temp.end());
    ll k=temp[0];
    ll ans=0;
    for(ll val:temp)
    {
        ans+=ceil((double)val/(double)k);
    }
    cout<<ans<<endl;
    return;
}
int main()
{
    ll n,m;cin>>n>>m;
    vector<ll>v(m);
    for(int i=0;i<m;i++)cin>>v[i];
    sort(v.begin(),v.end());
    if(m==n)
        cout<<0<<endl;
    else
    fun(v,n,m);
    return 0;
}
