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

//* ATCODER
#include<atcoder/all>
using namespace atcoder;
typedef modint998244353 mint;
//*/

/* BOOST MULTIPRECISION
#include<boost/multiprecision/cpp_int.hpp>
using namespace boost::multiprecision;
//*/

typedef long long ll;

#define rep(i, s, n) for (int i = (int)(s); i < (int)(n); i++)
#define rrep(i, s, n) for (int i = (int)(n)-1; i >= (int)(s); i--)

template <typename T> bool chmin(T &a, const T &b) {
	if (a <= b) return false;
	a = b;
	return true;
}

template <typename T> bool chmax(T &a, const T &b) {
	if (a >= b) return false;
	a = b;
	return true;
}

template <typename T> T max(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmax(ret, a[i]);
	return ret;
}

template <typename T> T min(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmin(ret, a[i]);
	return ret;
}

template <typename T> T sum(vector<T> &a){
	T ret = 0;
	for (int i=0; i<(int)a.size(); i++) ret += a[i];
	return ret;
}

int main(){
	int n; cin >> n;
	set<pair<int,int>> st;
	map<pair<int,int>,int> pts;
	vector<int> x(n), y(n);
	rep(i,0,n){
		cin >> x[i] >> y[i];
		pts[pair(x[i], y[i])]=i;
	}

	rep(i,0,n){
		for (auto [a, b]:
			{
				pair(x[i]-2, y[i]+1),
				pair(x[i]-2, y[i]-1),
				pair(x[i]+2, y[i]+1),
				pair(x[i]+2, y[i]-1),
				pair(x[i]-1, y[i]+2),
				pair(x[i]-1, y[i]-2),
				pair(x[i]+1, y[i]+2),
				pair(x[i]+1, y[i]-2)
			}
		){
			if (pts.find(pair(a, b)) != pts.end()) continue;
			st.insert(pair(a, b));
		}
	}

	vector<int> a, b;
	for (auto [i, j]: st){
		a.push_back(i);
		b.push_back(j);
	}

	int m = a.size();
	vector dp(m + 1, vector<int>(1 << n, 1e9));
	dp[0][0] = 0;

	rep(i,0,m){
		// HIGASHI
		vector<int> tar(4);
		if (pts.find(pair(a[i]+2, b[i]+1)) != pts.end()) tar[0] |= 1 << pts[pair(a[i]+2, b[i]+1)];
		if (pts.find(pair(a[i]+2, b[i]-1)) != pts.end()) tar[0] |= 1 << pts[pair(a[i]+2, b[i]-1)];
		if (pts.find(pair(a[i]-2, b[i]+1)) != pts.end()) tar[1] |= 1 << pts[pair(a[i]-2, b[i]+1)];
		if (pts.find(pair(a[i]-2, b[i]-1)) != pts.end()) tar[1] |= 1 << pts[pair(a[i]-2, b[i]-1)];
		if (pts.find(pair(a[i]-1, b[i]-2)) != pts.end()) tar[2] |= 1 << pts[pair(a[i]-1, b[i]-2)];
		if (pts.find(pair(a[i]+1, b[i]-2)) != pts.end()) tar[2] |= 1 << pts[pair(a[i]+1, b[i]-2)];
		if (pts.find(pair(a[i]-1, b[i]+2)) != pts.end()) tar[3] |= 1 << pts[pair(a[i]-1, b[i]+2)];
		if (pts.find(pair(a[i]+1, b[i]+2)) != pts.end()) tar[3] |= 1 << pts[pair(a[i]+1, b[i]+2)];
		rep(j,0,1<<n){
			chmin(dp[i+1][j], dp[i][j]); 
			rep(k,0,4){
				chmin(dp[i+1][j|tar[k]], dp[i][j] + 1);
			}
		}
	}

	if (dp[m][(1<<n)-1] > 1e8){
		cout << -1 << '\n';
	}else{
		cout << dp[m][(1<<n)-1] << '\n';
	}
}