#include<iostream>
#include<vector>
#include<algorithm>

using namespace std;
using ll = long long;
#include<queue>
#include<limits>

template< typename flow_t, typename cost_t >
struct PrimalDual {
  const cost_t INF;

  struct edge {
    int to;
    flow_t cap;
    cost_t cost;
    int rev;
    bool isrev;
  };
  vector< vector< edge > > graph;
  vector< cost_t > potential, min_cost;
  vector< int > prevv, preve;

  PrimalDual(int V) : graph(V), INF(numeric_limits< cost_t >::max()) {}

  void add_edge(int from, int to, flow_t cap, cost_t cost) {
    graph[from].emplace_back((edge) {to, cap, cost, (int) graph[to].size(), false});
    graph[to].emplace_back((edge) {from, 0, -cost, (int) graph[from].size() - 1, true});
  }

  cost_t min_cost_flow(int s, int t, flow_t f) {
    int V = (int) graph.size();
    cost_t ret = 0;
    using Pi = pair< cost_t, int >;
    priority_queue< Pi, vector< Pi >, greater< Pi > > que;
    potential.assign(V, 0);
    preve.assign(V, -1);
    prevv.assign(V, -1);

    while(f > 0) {
      min_cost.assign(V, INF);
      que.emplace(0, s);
      min_cost[s] = 0;
      while(!que.empty()) {
        Pi p = que.top();
        que.pop();
        if(min_cost[p.second] < p.first) continue;
        for(int i = 0; i < graph[p.second].size(); i++) {
          edge &e = graph[p.second][i];
          cost_t nextCost = min_cost[p.second] + e.cost + potential[p.second] - potential[e.to];
          if(e.cap > 0 && min_cost[e.to] > nextCost) {
            min_cost[e.to] = nextCost;
            prevv[e.to] = p.second, preve[e.to] = i;
            que.emplace(min_cost[e.to], e.to);
          }
        }
      }
      if(min_cost[t] == INF) return -1;
      for(int v = 0; v < V; v++) potential[v] += min_cost[v];
      flow_t addflow = f;
      for(int v = t; v != s; v = prevv[v]) {
        addflow = min(addflow, graph[prevv[v]][preve[v]].cap);
      }
      f -= addflow;
      ret += addflow * potential[t];
      for(int v = t; v != s; v = prevv[v]) {
        edge &e = graph[prevv[v]][preve[v]];
        e.cap -= addflow;
        graph[v][e.rev].cap += addflow;
      }
    }
    return ret;
  }

  void output() {
    for(int i = 0; i < graph.size(); i++) {
      for(auto &e : graph[i]) {
        if(e.isrev) continue;
        auto &rev_e = graph[e.to][e.rev];
        cout << i << "->" << e.to << " (flow: " << rev_e.cap << "/" << rev_e.cap + e.cap << ")" << endl;
      }
    }
  }
};

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    int h,w;
    cin>>h>>w;
    int m = h + w + 1;
    PrimalDual<ll,ll> g(2 + w + w);
    int st = w + w;
    int tt = st + 1;
    
    int dx[] = {-1,-1,0,0,1,1};
    int dy[] = {-1,0,-1,1,0,1};
    for(int i = 0;i<w;i++){
        g.add_edge(st,i,1,0);
        g.add_edge(i+w,tt,1,0);
        vector<vector<int>> vis(m,vector<int>(m,1e9));
        int x,y;
        cin>>x>>y;
        x--;y--;
        vis[x][y] = 0;
        vector<pair<int,int>> que;
        que.push_back(make_pair(x,y));
        for(int j = 0;j<que.size();j++){
            int ni = que[j].first;
            int nj = que[j].second;
            for(int k = 0;k<6;k++){
                int nni = ni + dx[k];
                int nnj = nj + dy[k];
                if(nni<0||nnj<0||nnj>=w+nni||nni>=m||nnj>=m) continue;
                if(vis[nni][nnj]<=vis[ni][nj]+1) continue;
                vis[nni][nnj] = vis[ni][nj] + 1;
                que.push_back(make_pair(nni,nnj));
            }
        }
        for(int j = 0;j<w;j++) g.add_edge(i,j+w,1,vis[0][j]);
    }
    cout<<g.min_cost_flow(st,tt,w)<<endl;
}