#include<bits/stdc++.h>
#include<atcoder/all>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef vector<int> vi;
typedef vector<long long> vl;
typedef vector<vector<int>> vvi;
typedef vector<vector<long long>> vvl;
typedef long double ld;
typedef pair<int, int> P;

ostream& operator<<(ostream& os, const modint& a) {os << a.val(); return os;}
template <int m> ostream& operator<<(ostream& os, const static_modint<m>& a) {os << a.val(); return os;}
template <int m> ostream& operator<<(ostream& os, const dynamic_modint<m>& a) {os << a.val(); return os;}
template<typename T> istream& operator>>(istream& is, vector<T>& v){int n = v.size(); assert(n > 0); rep(i, n) is >> v[i]; return is;}
template<typename U, typename T> ostream& operator<<(ostream& os, const pair<U, T>& p){os << p.first << ' ' << p.second; return os;}
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : " "); return os;}
template<typename T> ostream& operator<<(ostream& os, const vector<vector<T>>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : ""); return os;}

template<typename T> void chmin(T& a, T b){a = min(a, b);}
template<typename T> void chmax(T& a, T b){a = max(a, b);}

int main(){
	int n;
	cin >> n;
	vector<int> x(n), y(n);
	rep(i, n) cin >> x[i] >> y[i];
	map<pair<int, int>, vector<vector<int>>> ma;
	vector<tuple<int, int, int>> v = {{0, 2, 1}, {0, 2, -1}, {1, 1, 2}, {1, -1, 2}, {2, -2, 1}, {2, -2, -1}, {3, 1, -2}, {3, -1, -2}};
	rep(i, n){
		for(auto [dir, dy, dx] : v){
			pair<int, int> p = {y[i] + dy, x[i] + dx};
			if(ma.find(p) == ma.end()) ma[p].resize(4);
			ma[p][dir].push_back(i);
		}
	}
	{
		vector<pair<int, int>> del;
		rep(i, n){
			pair<int, int> p = {y[i], x[i]};
			if(ma.find(p) == ma.end()) continue;
			del.push_back(p);
		}
		for(auto p : del) ma.erase(p);
	}
	const int INF = 1001001;
	vector<int> dp(1 << n, INF);
	dp[0] = 0;
	for(auto& [p, vec] : ma){
		vector<int> dp_old(1 << n, INF);
		swap(dp, dp_old);
		rep(bit, 1 << n){
			int bit_new = bit;
			rep(dir, 4)	for(auto idx : vec[dir]) bit_new |= (1 << idx);
			chmin(dp[bit_new], dp_old[bit] + 1);
			chmin(dp[bit], dp_old[bit]);
		}
	}
	int ans = dp[(1 << n) -1];
	if(ans >= INF) cout << "-1\n";
	else cout << ans << "\n";
	return 0;
}