#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll,ll> pi;
typedef vector<ll> vi;
typedef vector<pi> vpi;
typedef long double ld;
const int inf = 1e9 + 7;
const int mod = 998244353;
const int maxn = 1e6 + 3;
#define pb push_back
#define mp make_pair
#define ff first
#define ss second
#define rep(i, a, b) for(int i = a; i < b; i++)
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
#define lb lower_bound
#define ub upper_bound
#define fact(n) rep(i, 1, n+1)ft.pb((ft[i-1]*i)%mod);
#define mod_in(a) exp(a, mod-2)
#define ncr(n, r) ((ft[n]*mod_in((ft[r]*ft[(n)-(r)])%mod))%mod)
#define deb(x) cout << #x << " " << x << endl;
#define flash ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define endl '\n'

/*#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
template<typename T>
using ordered_set = tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;
template<typename T>
using ordered_multiset = tree<T,null_type,less_equal<T>,rb_tree_tag,tree_order_statistics_node_update>;*/

ll prime[4000005];
ll exp(ll a,ll b);
void sieve(ll n);
ll gcd(ll a ,ll b);
ll countdigit(ll n);
vector<ll>ft;
int main()
{   
    flash;
    ll t=1;
   // sieve(4000003);
    //ft.pb(1);
    //fact(10000001);
    //cin >> t;
        ll n,m;
        cin >> n >> m;

        if (m == 0) {
            cout << 1;
            return 0;
        }
        ll a[m];
        
        rep(i,0,m){
            cin>>a[i];
        }
        sort(a,a+m);
        
        vector<ll>gaps;
        
        rep(i,1,m){
            gaps.pb(a[i]-a[i-1]-1);
        }
        gaps.pb(n-a[m-1]);
        
        gaps.pb(a[0]-1);
        
        ll ans = n;
        
    if (!gaps.empty()) for (auto i : gaps) {
        if (i > 0) 
            ans = min(ans, i);
    }
    ll count = 0;
    if (!gaps.empty()) for (auto i : gaps) {
        if (i > 0)
            count += (i/ans + (i%ans != 0));
    }
    cout << count;
       
        
    
}

//
//
//
//
//
// IMPORTANT FUNCTIONS
ll exp(ll a, ll b) {
	int res=1;
	while(b) {
		if(b&1)
			res=(res*a)%mod;
		a=(a*a)%mod;
		b>>=1;
	}
	return res%mod;
}
ll gcd(ll a,ll b){
    if(a==0)return b;
    return gcd(b%a,a);
}
void sieve(ll n) {
    for(ll i=0;i<=n;i++) {
        prime[i]=1;
    }
    prime[0]=0;prime[1]=0;
    for(ll i=2;i<=n;i++) {
        if(prime[i]==1) {
            for(ll j=2;i*j<=n;j++) {
                prime[i*j]=0;
            }
        }
    }
}
ll countdigit(ll  n)
{
    return floor(log10(n) + 1);
}
#ifdef DEBUG
#define _GLIBCXX_DEBUG
#endif

#include "bits/stdc++.h"
using namespace std;

#define all(x) (x).begin(), (x).end()
#define IOS ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define un_map unordered_map
#define rep(i,a,n) for (int i = a; i < n; ++i) 
#define rrep(i,a,n) for (int i = a; i >= n; --i) 
#define irep(i,a,n,inc) for (int i = a; i < n; i += inc) 
#define endl "\n"

typedef long long ll;

void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
 
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
 
#ifndef ONLINE_JUDGE
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif

#define int long long 

long long POW(long long a, long long b) {
	long long ans = 1;
	while (b) {
			if(b & 1) {
					ans = ans * a;
			}
			a = a * a;
			b /= 2;
	}
	return ans;
}

int const nax = 2e5 + 11;
int a[nax];
int n, m;

int32_t main() {
  IOS;
  cin >> n >> m;
  for (int i = 1; i <= m; ++i) cin >> a[i];
  a[m + 1] = n;
  sort(a, a + m + 2);
  int minn = 1e18;
  for (int i = 1; i < m + 2; ++i) {
		int x = a[i] - a[i - 1];
		if (i != m + 1) {
			if (x > 1) {
				minn = min(minn, x - 1);
			}
		}else {
			if (x) {
				minn = min(minn, x);
			}
		}
	}
	//debug(minn);
	if (minn == 1e18) {
		cout << 0;
		return 0;
	}
	int ans = 0;
	for (int i = 1; i < m + 2; ++i) {
		int y = a[i] - a[i - 1];
		if (i != m + 1) {
			ans += (y + minn - 2) / minn;
		}else {
			ans += (y + minn - 1) / minn;
		}
	}
	cout << ans;
}











