#include<bits/stdc++.h>
#define ll long long 
#define mp make_pair 
#define f(i,n) for(int i=0;i<n;i++) 
#define F first 
#define S second 
#define pb push_back 

using namespace std;

void test(){
	ll n,m;
	cin>>n>>m;
	
	if(m==0){
		cout<<"1\n";
		return ;
	}
	
	vector<ll> a(m);
	f(i,m)cin>>a[i];
	sort(a.begin(),a.end());
	if(a[m-1]!=n)a.pb(n+1);
	if(a[0]!=1)a.pb(0);
	sort(a.begin(),a.end());
	
	vector<ll> v;
	
	f(i,a.size()-1){
		if(a[i+1]-a[i]-1!=0)
			v.pb(a[i+1]-a[i]-1);
	}
	if(v.size()==0){
		cout<<"0\n";
		return ;
	}
	sort(v.begin(),v.end());
	
	ll dif = v[0];
	ll res = 0;
	f(i,v.size()){
		res = res + (v[i]/dif);
		if(v[i]%dif!=0)res++;
	}
	cout<<res<<"\n";
	
}

int main(){
	std::ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	int tests=1;
	// cin>>tests;
	while(tests--){
		test();
	}
}