#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")

#include <algorithm>
#include <atcoder/all>
#include <bitset>
#include <cassert>
#include <cmath>
#include <ctime>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;
using namespace atcoder;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define repk(i, k, n) for (int i = k; i < (int)(n); i++)
#define all(v) v.begin(), v.end()
#define mod1 1000000007
#define mod2 998244353
#define mod3 100000007
#define vi vector<int>
#define vs vector<string>
#define vc vector<char>
#define vl vector<ll>
#define vb vector<bool>
#define vvi vector<vector<int>>
#define vvc vector<vector<char>>
#define vvl vector<vector<ll>>
#define vvb vector<vector<bool>>
#define vvvi vector<vector<vector<int>>>
#define vvvl vector<vector<vector<ll>>>
#define pii pair<int, int>
#define pil pair<int, ll>
#define pli pair<ll, int>
#define pll pair<ll, ll>
#define vpii vector<pair<int, int>>
#define vpll vector<pair<ll, ll>>
#define vvpii vector<vector<pair<int, int>>>
#define vvpll vector<vector<pair<ll, ll>>>

using mint = modint998244353;

template <typename T>
void debug(T e) {
    cerr << e << endl;
}

template <typename T>
void debug(vector<T> &v) {
    rep(i, v.size()) { cerr << v[i] << " "; }
    cerr << endl;
}

template <typename T>
void debug(vector<vector<T>> &v) {
    rep(i, v.size()) {
        rep(j, v[i].size()) { cerr << v[i][j] << " "; }
        cerr << endl;
    }
}

template <typename T>
void debug(vector<pair<T, T>> &v) {
    rep(i, v.size()) { cerr << v[i].first << " " << v[i].second << endl; }
}

template <typename T>
void debug(set<T> &st) {
    for (auto itr = st.begin(); itr != st.end(); itr++) {
        cerr << *itr << " ";
    }
    cerr << endl;
}

template <typename T>
void debug(multiset<T> &ms) {
    for (auto itr = ms.begin(); itr != ms.end(); itr++) {
        cerr << *itr << " ";
    }
    cerr << endl;
}

template <typename T>
void debug(map<T, T> &mp) {
    for (auto itr = mp.begin(); itr != mp.end(); itr++) {
        cerr << itr->first << " " << itr->second << endl;
    }
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
    cerr << H << " ";
    debug_out(T...);
}

ll INF = 1000000000;

int main() {
    ll H, W;
    cin >> H >> W;
    vector<ll> a(W);
    vector<ll> b(W);
    for (ll i = 0; i < W; i++) {
        cin >> a[i] >> b[i];
        a[i]--;
        b[i]--;
    }
    queue<pll> que;
    vector<vector<ll>> dists(W, vector<ll>(W));
    vector<vector<ll>> dist(H, vector<ll>(H + W, INF));
    vector<ll> dx = {-1, -1, 0, 0, 1, 1};
    vector<ll> dy = {-1, 0, -1, 1, 0, 1};
    for (ll i = 0; i < W; i++) {
        for (ll j = 0; j < H; j++) {
            for (ll k = 0; k < H + W; k++) {
                dist[j][k] = INF;
            }
        }
        dist[a[i]][b[i]] = 0;
        que.push(pll(a[i], b[i]));
        while (que.size()) {
            pll p = que.front();
            que.pop();
            ll px = p.first;
            ll py = p.second;
            for (ll j = 0; j < 6; j++) {
                if (px + dx[j] >= 0 && px + dx[j] < H && py + dy[j] >= 0 &&
                    py + dy[j] < W + px + dx[j]) {
                    if (dist[px + dx[j]][py + dy[j]] < INF) {
                        continue;
                    }
                    dist[px + dx[j]][py + dy[j]] = dist[px][py] + 1;
                    que.push(pll(px + dx[j], py + dy[j]));
                }
            }
        }
        // debug(dist);
        for (ll j = 0; j < W; j++) {
            dists[i][j] = dist[0][j];
        }
    }
    debug(dists);
    mcf_graph<ll, ll> graph(2 * W + 2);
    for (ll i = 0; i < W; i++) {
        for (ll j = 0; j < W; j++) {
            graph.add_edge(i, W + j, 1, dists[i][j]);
        }
    }
    for (ll i = 0; i < W; i++) {
        graph.add_edge(2 * W, i, 1, 0);
    }
    for (ll i = 0; i < W; i++) {
        graph.add_edge(W + i, 2 * W + 1, 1, 0);
    }
    pll flp = graph.flow(2 * W, 2 * W + 1);
    cout << flp.second << endl;
}