using System;
using System.Collections.Generic;
using System.Collections;
using System.Collections.Specialized;
using System.Linq;
using System.Text;
using System.IO;
using System.Reflection;
using static System.Math;
using System.Numerics;
static class Program{
	const int mod=(int)1e9+7;
	static void Main(){
		Sc sc=new Sc();
		var s=sc.Ia;
		int g=s[0]+s[1]*3+1;
		var fl=new Mfp(g+1);
		for(int i = 0;i<s[1];i++) {
			if(i!=0){fl.Edge(s[0]+i*3+1,s[0]+(i-1)*3+1,1);}
			if(i!=s[1]-1){fl.Edge(s[0]+i*3+2,s[0]+(i+1)*3+2,1);}
			fl.Edge(s[0]+i*3+1,s[0]+i*3,1);
			fl.Edge(s[0]+i*3+2,s[0]+i*3,1);
			fl.Edge(s[0]+i*3,g,1);
		}
		for(int i = 0;i<s[0];i++) {
			var e=sc.Ia;
			fl.Edge(g-1,i,1);
			if(e[0]!=0){fl.Edge(i,s[0]+(e[0]-1)*3+1,1);}
			if(e[1]!=s[1]+1){fl.Edge(i,s[0]+(e[1]-1)*3+2,1);}
		}
		Console.WriteLine("{0}",s[0]-fl.Dinic(g-1,g));
	}
}
public class Mfp{
	private List<int>[] li;
	private Dictionary<int,int>[] h;
	private bool[] b;
	private int[] l;
	private int n,ans,go;
	private const int inf=int.MaxValue;
	public Mfp(int n){
		this.n=n;
		li=new List<int>[n];
		h=new Dictionary<int,int>[n+1];
		for(int i=0;i<n;i++){
			li[i]=new List<int>();
			h[i]=new Dictionary<int,int>();
		}
	}
	public void Edge(int a,int b,int c){
		if(a==b){return;}
		if(!h[a].ContainsKey(b)){
			li[a].Add(b);
			li[b].Add(a);
			h[a].Add(b,c);
			h[b].Add(a,0);
		}
		else{h[a][b]+=c;}
	}
	public int Dinic(int st,int go){
		this.go=st;
		ans=0;
		int z=1;
		while(z>0){
			l=new int[n];
			var qu=new Queue<Tuple<int,int>>();
			qu.Enqueue(Tuple.Create(st,1));
			l[st]=1;
			while(qu.Count>0){
				var e=qu.Dequeue();
				for(int i=0;i<li[e.Item1].Count;i++){
					if(l[li[e.Item1][i]]==0&&h[e.Item1][li[e.Item1][i]]!=0){
						l[li[e.Item1][i]]=e.Item2+1;
						qu.Enqueue(Tuple.Create(li[e.Item1][i],e.Item2+1));
					}
				}
				if(e.Item1==go){break;}
			}
			b=new bool[n];
			z=Fu3(go,inf);
			ans+=z;
		}
		return ans;
	}
	private int Fu3(int a,int p){
		if(a==go){return p;}
		int u=0;
		for(int i=0;i<li[a].Count&&p>u;i++){
			if(!b[li[a][i]]&&l[li[a][i]]<l[a]&&h[li[a][i]][a]!=0){
				int z=Fu3(li[a][i],Min(p-u,h[li[a][i]][a]));
				h[li[a][i]][a]-=z;
				h[a][li[a][i]]+=z;
				u+=z;
			}
		}
		if(u==0){b[a]=true;}
		return u;
	}
	public int Ff(int st,int go){
		this.go=go;
		ans=0;
		int z=1;
		while(z>0){
			b=new bool[n];
			z=Fu2(st,inf);
			ans+=z;
		}
		return ans;
	}
	private int Fu2(int a,int p){
		if(a==go){return p;}
		int u=0;
		b[a]=true;
		for(int i=0;i<li[a].Count&&p>u;i++){
			if(!b[li[a][i]]&&h[a][li[a][i]]!=0){
				int z=Fu2(li[a][i],Min(p-u,h[a][li[a][i]]));
				h[a][li[a][i]]-=z;
				h[li[a][i]][a]+=z;
				u+=z;
			}
		}
		if(u!=0){b[a]=false;}
		return u;
	}
	public void Vc(int s,Action<int> f){
		b=new bool[n];
		Fu4(s,0,f);
		for(int i=0;i<li[s].Count;i++){
			if(h[s][li[s][i]]==0&&!b[li[s][i]]){f(li[s][i]);}
		}
	}
	private void Fu4(int a,int p,Action<int> f){
		b[a]=true;
		if((p&1)==0&&p!=0){f(a);}
		for(int i=0;i<li[a].Count;i++){
			if(h[a][li[a][i]]!=0&&!b[li[a][i]]){Fu4(li[a][i],p+1,f);}
		}
	}
}

public class Sc{
	public int I{get{return int.Parse(Console.ReadLine());}}
	public long L{get{return long.Parse(Console.ReadLine());}}
	public double D{get{return double.Parse(Console.ReadLine());}}
	public string S{get{return Console.ReadLine();}}
	public int[] Ia{get{return Array.ConvertAll(Console.ReadLine().Split(),int.Parse);}}
	public long[] La{get{return Array.ConvertAll(Console.ReadLine().Split(),long.Parse);}}
	public double[] Da{get{return Array.ConvertAll(Console.ReadLine().Split(),double.Parse);}}
	public string[] Sa{get{return Console.ReadLine().Split();}}
	public object[] Oa{get{return Console.ReadLine().Split();}}
	public int[] Ia2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),int.Parse);}}
	public int[] Ia3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),int.Parse);}
	public int[] Ia3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),int.Parse);}
	public long[] La2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),long.Parse);}}
	public long[] La3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),long.Parse);}
	public long[] La3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),long.Parse);}
	public T[] Arr<T>(int n,Func<T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f();}return a;}
	public T[] Arr<T>(int n,Func<int,T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i);}return a;}
	public T[] Arr<T>(int n,Func<string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(Console.ReadLine().Split());}return a;}
	public T[] Arr<T>(int n,Func<int,string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i,Console.ReadLine().Split());}return a;}
}