#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 4000000000000000001


int main(){
	int H,W;
	cin>>H>>W;
	vector<pair<int,int>> p;
	rep(i,H){
		rep(j,W+i){
			p.emplace_back(i,j);
		}
	}
	mcf_graph<int,int> G(p.size()+2);
	int S = p.size(),T = p.size()+1;
	vector<int> dx = {-1,-1,0,0,1,1},dy = {-1,0,-1,1,0,1};
	rep(i,p.size()){
		rep(j,6){
			int ii = p[i].first + dx[j],jj = p[i].second + dy[j];
			int d = distance(p.begin(),lower_bound(p.begin(),p.end(),make_pair(ii,jj)));
			if(d==p.size() || p[d]!=make_pair(ii,jj))continue;
			G.add_edge(i,d,10000,1);
		}
	}
	rep(i,W)G.add_edge(i,T,1,0);
	rep(i,W){
		int x,y;
		cin>>x>>y;
		x--,y--;
		int d = distance(p.begin(),lower_bound(p.begin(),p.end(),make_pair(x,y)));
		G.add_edge(S,d,1,0);
	}
	
	cout<<G.flow(S,T).second<<endl;
	
	return 0;
}