#include <bits/stdc++.h>
using namespace std;
#define rep(i, n)  for(long long i=0;i<(long long)(n);i++)
#define REP(i,k,n) for(long long i=k;i<(long long)(n);i++)
#define all(a) a.begin(),a.end()
#define pb push_back
#define eb emplace_back
#define lb(v,k) (lower_bound(all(v),k)-v.begin())
#define ub(v,k) (upper_bound(all(v),k)-v.begin())
#define fi first
#define se second
#define pi M_PI
#define PQ(T) priority_queue<T>
#define SPQ(T) priority_queue<T,vector<T>,greater<T>>
#define dame(a) {out(a);return 0;}
#define decimal cout<<fixed<<setprecision(15);
typedef long long ll;
typedef pair<ll,ll> P;
typedef tuple<ll,ll,ll> PP;
typedef tuple<ll,ll,ll,ll> PPP;
typedef multiset<ll> S;
using vi=vector<ll>;
using vvi=vector<vi>;
using vvvi=vector<vvi>;
using vp=vector<P>;
using vvp=vector<vp>;
using vb=vector<bool>;
using vvb=vector<vb>;
const ll inf=1001001001001001;
const int INF=1001001001;
const int mod=1000000007;
const double eps=1e-10;
template<class T> bool chmin(T&a,T b){if(a>b){a=b;return true;}return false;}
template<class T> bool chmax(T&a,T b){if(a<b){a=b;return true;}return false;}
template<class T> void out(T a){cout<<a<<'\n';}
template<class T> void outp(T a){cout<<'('<<a.fi<<','<<a.se<<')'<<'\n';}
template<class T> void outvp(T v){rep(i,v.size())cout<<'('<<v[i].fi<<','<<v[i].se<<')';cout<<'\n';}
template<class T> void outvvp(T v){rep(i,v.size())outvp(v[i]);}
template<class T> void outv(T v){rep(i,v.size()){if(i)cout<<' ';cout<<v[i];}cout<<'\n';}
template<class T> void outvv(T v){rep(i,v.size())outv(v[i]);}
template<class T> bool isin(T x,T l,T r){return (l)<=(x)&&(x)<=(r);}
template<class T> void yesno(T b){if(b)out("yes");else out("no");}
template<class T> void YesNo(T b){if(b)out("Yes");else out("No");}
template<class T> void YESNO(T b){if(b)out("YES");else out("NO");}
template<class T> void noyes(T b){if(b)out("no");else out("yes");}
template<class T> void NoYes(T b){if(b)out("No");else out("Yes");}
template<class T> void NOYES(T b){if(b)out("NO");else out("YES");}
void outs(ll a,ll b){if(a>=inf-100)out(b);else out(a);}
ll gcd(ll a,ll b){if(b==0)return a;return gcd(b,a%b);}
ll modpow(ll a,ll b){a%=mod;if(b==0)return 1;if(b&1)return a*modpow(a,b-1)%mod;ll k=modpow(a,b/2);return k*k%mod;}
vi level,memo;
vvi v;
ll n;
void bfs(ll s){
    level=vi(n,inf);
    queue<P> q;
    q.push(P(s,0));
    level[s]=0;
    while(!q.empty()){
        auto t=q.front();q.pop();
        rep(i,n)if(v[t.fi][i]){
            if(chmin(level[i],t.se+1))q.push(P(i,t.se+1));
        }
    }
}
ll dfs(ll i,ll t,ll f){
    if(i==t)return f;
    REP(j,memo[i],n){
        memo[i]++;
        if(v[i][j]==0||level[i]>=level[j])continue;
        ll d=dfs(j,t,min(f,v[i][j]));
        if(d>0){
            v[i][j]-=d;
            v[j][i]+=d;
            return d;
        }
    }
    return 0;
}
ll max_flow(ll s,ll t){
    ll flow=0;
    while(true){
        bfs(s);
        if(level[t]==inf)return flow;
        memo=vi(n);
        while(true){
            ll f=dfs(s,t,inf);
            if(f==0)break;
            flow+=f;
        }
    }
    return flow;
}
int main(){
    cin>>n;
    v=vvi(n*2+2,vi(n*2+2));
    vp point(2*n);
    rep(i,2*n)cin>>point[i].fi>>point[i].se;
    rep(i,n)rep(j,n)if(point[i].fi<=point[j+n].fi&&point[i].se<=point[j+n].se){
        v[i+1][j+n+1]++;
    }
    rep(i,n)v[0][i+1]++;
    rep(i,n)v[i+n+1][n*2+1]++;
    n=n*2+2;
    outs(max_flow(0,n-1),-1);
}

Measured memory_bytes: 640