#include <bits/stdc++.h>
#include <atcoder/all>
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#define SELECTER(_1,_2,_3,SELECT,...) SELECT
#define rep1(i,n) for(int i=0;i<int(n);++i)
#define rep2(i,a,n) for(int i=int(a);i<int(n);++i)
#define rep(...) SELECTER(__VA_ARGS__,rep2,rep1)(__VA_ARGS__)
#define RSELECTER(_1, _2, _3, RSELECT, ...) RSELECT
#define rrep1(i,n) for(int i=(int)(n)-1;i>=0;--i)
#define rrep2(i,a,n) for(int i=(int)(n)-1;i>=(int)a;--i)
#define rrep(...) RSELECTER(__VA_ARGS__, rrep2, rrep1)(__VA_ARGS__)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define fi first
#define se second
#define PrintR LogOutput
#ifdef _DEBUG
#define Log(...) LogOutput(__VA_ARGS__)
#else
#define Log(...)
#endif
#define M_PI 3.14159265358979323846
using namespace std;
using namespace atcoder;
using ll=long long;
using ld=long double;
using pii=pair<int,int>;
using pll=pair<long long,long long>;
using pdd=pair<long double,long double>;
using tp=tuple<int,int,int>;
using tpll=tuple<ll,ll,ll>;
using veci=vector<int>;
using vecpii=vector<pair<int,int>>;
using vecll=vector<long long>;
using vecpll=vector<pair<long long,long long>>;
using vecpdd=vector<pair<long double,long double>>;
using vecs=vector<string>;
using vecb=vector<bool>;
using vecd=vector<long double>;
using vectp=vector<tp>;
using vectpll=vector<tpll>;
using mint=modint998244353;
using mint10=modint1000000007;
template<typename T, typename S>
istream& operator>>(istream& in, pair<T, S>& a){return in >> a.first >> a.second;}
template<typename T, typename S>
ostream& operator<<(ostream& out, const pair<T, S>& a){return out << a.first << ' ' << a.second;}
ostream& operator<<(ostream& out, const mint& a){return out << a.val();}
ostream& operator<<(ostream& out, const mint10& a){return out << a.val();}
ostream& operator<<(ostream& out, const modint& a){return out << a.val();}
template<typename T>
ostream& operator<<(ostream& out, const vector<T>& d){for(int i = 0 ; i < d.size() ; ++i) out << d[i] << (i == d.size() - 1 ? "" : " "); return out;}
template<typename T, typename S>
pair<T, S> operator+(const pair<T, S>& a, const pair<T, S>& b){return {a.fi + b.fi, a.se + b.se};}
template<typename T, typename S>
pair<T, S> operator-(const pair<T, S>& a, const pair<T, S>& b){return {a.fi - b.fi, a.se - b.se};}
template<class T> inline bool chmax(T& a,T b){if(a<b) {a=b;return true;} return false;}
template<class T> inline bool chmin(T& a,T b){if(a>b) {a=b;return true;} return false;}
bool Judge(int i, int j, int h, int w){return i < 0 || j < 0 || i >= h || j >= w;}
bool PrintA(int i){cout<<(i ? "Yes" : "No")<<endl;return i;}

constexpr ll INF=numeric_limits<ll>::max() >> 2;
constexpr int inf=numeric_limits<int>::max() >> 1;
constexpr ll MOD=998244353;
const int vi[] = {0, 1, 0, -1}, vj[] = {1, 0, -1, 0};

template<typename... Args>
void LogOutput(Args&&... args){
	stringstream ss;
	((ss << args << ' '), ...);
	cout << ss.str().substr(0, ss.str().length() - 1) << endl;
}
template<typename T>
void LogOutput(vector<vector<T>>& data){for(auto d : data) LogOutput(d);}

int main(){
	ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int h, w;cin>>h>>w;
	vecpii a(w);
	rep(i, w) cin>>a[i], a[i].fi--, a[i].se--;
	auto f = [&](pii a){
		auto [l, r] = a;
		vector dist(h, veci(h + w, -1));
		dist[l][r] = 0;
		queue<pii> q;
		q.emplace(l, r);
		while(q.size()){
			auto [i, j] = q.front(); q.pop();
			rep(l, -1, 2) rep(r, -1, 2){
				int ni = i + l, nj = j + r;
				if(Judge(ni, nj, h, h + w) || l * r < 0) continue;
				if(dist[ni][nj] == -1){
					dist[ni][nj] = dist[i][j] + 1;
					q.emplace(ni, nj);
				}
			}
		}
		return dist;
	};
	vector<vector<veci>> d(w);
	rep(i, w) d[i] = f(a[i]);
	int sr = w * 2, tr = sr + 1;
	mcf_graph<int, int> g(tr + 1);
	rep(i, w) rep(j, w) g.add_edge(i, j + w, 1, d[i][0][j]);
	rep(i, w) g.add_edge(sr, i, 1, 0), g.add_edge(i + w, tr, 1, 0);
	cout<<g.flow(sr, tr).se<<endl;
}