#include <iostream>
#include <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <cstdio>
#include <cmath>
#include <tuple>
#define int long long
#define rep(i, n) for(i = 0; i < n; i++)
using namespace std;

int n, m;
int a[200002];

signed main() {
	int i;
	
	cin >> n >> m;
	rep(i, m) cin >> a[i];
	a[m] = 0; a[m + 1] = n + 1;
	m += 2;
	sort(a, a + m);
	
	int k = n;
	rep(i, m - 1) {
		if (a[i + 1] - a[i] >= 2) {
			k = min(k, a[i + 1] - a[i] - 1);
		}
	}
	
	int ans = 0;
	rep(i, m - 1) {
		if (a[i + 1] - a[i] >= 2) {
			ans += ((a[i + 1] - a[i] - 1) + k - 1) / k;
		}
	}
	
	cout << ans << endl;
	return 0;
}
#include<bits/stdc++.h>
using namespace std; 
using ll = long long;

set<int> getFactors(ll n) {
    set<int> st;
    for (ll i = 1; i * i <= n; ++i) {
        if (n % i == 0) {
            st.insert(i);
            st.insert(n / i);
        }
    }
    return st;
}

void solve() {
    int N, M; cin >> N >> M;
    vector<int> A (M + 2);
    A[0] = 0;
    A[M + 1] = N + 1;
    for (int i = 1; i < M + 1; ++i) cin >> A[i];
    sort( A.begin(), A.end() );

    set<int> factors;
    for (int i = 1; i < M + 2; ++i) {
        int diff = A[i] - A[i - 1] - 1;
        if (diff > 0)
            factors.emplace(diff);
    }


    int ans = INT_MAX;

    int f = *factors.begin();
    int result = 0;
    for (int i = 1; i < M + 2; ++i) {
        int diff = A[i] - A[i - 1] - 1;
        if (diff <= 0) continue;
        if (diff % f != 0) { result++; }
        result += diff / f;
    }
    ans = min(ans, result);

    cout << ans << endl;
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int t = 1;
    while(t--) {
        solve();
    }
}
