#include <bits/stdc++.h>
using namespace std;
#define int long long

signed main(){
    // cout << fixed << setprecision(10) << flush;

    int n;
    cin >> n;

    vector<vector<int>> x(n, vector<int>());
    vector<vector<int>> y(n, vector<int>());
    for(int i=0; i<n; i++){
        int a;
        cin >> a;
        for(int j=0; j<a; j++){
            int xx, yy;
            cin >> xx >> yy;
            x[i].push_back(xx-1);
            y[i].push_back(yy);
        }
    }

    int max_pop = 0;
    for(int i=0; i<(1<<n); i++){
        bool flag = true;
        for(int j=0; j<n; j++){
            if((i & (1<<j)) == (1<<j)){
                // j が正直者のとき
                for(int k=0; k<x[j].size(); k++){
                    if((i & (1<<x[j][k])) != y[j][k] * (1<<x[j][k])){
                        // 証言と違う
                        flag = false;
                    }
                }
            }
        }
        if(flag && max_pop < __builtin_popcount(i)){
            max_pop = __builtin_popcount(i);
        }
    }
    cout << max_pop << endl;

    return 0;
}

#include<bits/stdc++.h>
using namespace std;
#define inf 1000000000
#define INF 1000000000000000
#define ll long long
#define ull unsigned long long
#define M (int)(1e9+7)
#define P pair<int,int>
#define PLL pair<ll,ll>
#define FOR(i,m,n) for(int i=(int)m;i<(int)n;i++)
#define RFOR(i,m,n) for(int i=(int)m;i>=(int)n;i--)
#define rep(i,n) FOR(i,0,n)
#define rrep(i,n) RFOR(i,n,0)
#define all(a) a.begin(),a.end()
#define IN(a,n) rep(i,n){ cin>>a[i]; }
const int vx[4] = {0,1,0,-1};
const int vy[4] = {1,0,-1,0};
#define PI 3.14159265
#define F first
#define S second
#define PB push_back
#define EB emplace_back
#define int ll
void init(){
  cin.tie(0);
  ios::sync_with_stdio(false);
}




template< typename T >
struct edge {
  int src, to;
  T cost;

  edge(int to, T cost) : src(-1), to(to), cost(cost) {}

  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }
};

template< typename T >
using Edges = vector< edge< T > >;
template< typename T >
using WG = vector< Edges< T > >;
using UG = vector< vector< int > >;
template< typename T >
using Matrix = vector< vector< T > >;



struct UnionFind
{

  vector<int> par;
  vector<int> sizes;

  UnionFind(int n) : par(n), sizes(n, 1) {
    rep(i,n) par[i] = i;
  }

  int find(int x) {
    if (x == par[x]) return x;
    return par[x] = find(par[x]); 
  }

  bool unite(int x, int y) {

    x = find(x);
    y = find(y);

    if (x == y) return false;

    if (sizes[x] < sizes[y]) swap(x, y);

    par[y] = x;
    sizes[x] += sizes[y];

    return true;
  }

  int size(int x) {
    return sizes[find(x)];
  }
};


template<typename T>
T kruskal(Edges<T> &es,int n) {
  sort(all(es),[](const edge<T> & a,const edge<T> &b){
      return (a.cost < b.cost);
      });

  UnionFind uf(n);
  T min_cost = 0;

  for(auto &e:es) {
    if(uf.unite(e.src,e.to)) 
      min_cost += e.cost;
  }

  return min_cost;
}



main(){
  int n;
  cin>>n;
  vector<P> x,y;

  rep(i,n){
    int a,b;
    cin>>a>>b;
    x.PB(P(a,i));
    y.PB(P(b,i));
  }

  sort(all(x));
  sort(all(y));

  Edges<int> es;
  rep(i,n-1){
    P p=x[i+1],q=x[i];
    es.PB(edge<int>(p.S,q.S,abs(p.F-q.F)));
    es.PB(edge<int>(q.S,p.S,abs(p.F-q.F)));
    p=y[i+1],q=y[i];
    es.PB(edge<int>(p.S,q.S,abs(p.F-q.F)));
    es.PB(edge<int>(q.S,p.S,abs(p.F-q.F)));
  }

  cout<<kruskal(es,n)<<endl;
}
