#include<bits/stdc++.h>
#include<atcoder/all>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef vector<int> vi;
typedef vector<long long> vl;
typedef vector<vector<int>> vvi;
typedef vector<vector<long long>> vvl;
typedef long double ld;
typedef pair<int, int> P;

ostream& operator<<(ostream& os, const modint& a) {os << a.val(); return os;}
template <int m> ostream& operator<<(ostream& os, const static_modint<m>& a) {os << a.val(); return os;}
template<typename T> istream& operator>>(istream& is, vector<T>& v){int n = v.size(); assert(n > 0); rep(i, n) is >> v[i]; return is;}
template<typename U, typename T> ostream& operator<<(ostream& os, const pair<U, T>& p){os << p.first << ' ' << p.second; return os;}
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : " "); return os;}
template <typename T> ostream& operator<<(ostream& os, const vector<vector<T>>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : ""); return os;}

int main(){
	int h, w;
	cin >> h >> w;
	int n = h * (h + w);
	mcf_graph<int, int> graph(n + 2);
	int sv = n;
	int tv = n + 1;
	rep(y, h) rep(x, h + w){
		int from = y * (h + w) + x;
		if(y < h - 1){
			int to = (y + 1) * (h + w) + x;
			graph.add_edge(from, to, 100, 1);
			graph.add_edge(to, from, 100, 1);
		}
		if(x < h + w - 1){
			int to = y * (h + w) + (x + 1);
			graph.add_edge(from, to, 100, 1);
			graph.add_edge(to, from, 100, 1);
		}
		if(y < h - 1 and x < h + w - 1){
			int to = (y + 1) * (h + w) + (x + 1);
			graph.add_edge(from, to, 100, 1);
			graph.add_edge(to, from, 100, 1);
		}
	}
	rep(i, w){
		int y, x;
		cin >> y >> x;
		x--; y--;
		graph.add_edge(sv, y * (h + w) + x, 1, 0);
	}
	rep(x, w){
		graph.add_edge(x, tv, 1, 0);
	}
	auto [cap, cost] = graph.flow(sv, tv);
//	cout << cap << ' ' << cost << "\n";
	cout << cost << "\n";
	return 0;
}