#include <iostream>
#include <vector>
#include <algorithm>
#include <array>
#include <iterator>
#include <string>
#include <cctype>
#include <cstring>
#include <cstdlib>
#include <cassert>
#include <cmath>
#include <ctime>
#include <iomanip>
#include <numeric>
#include <stack>
#include <queue>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <bitset>
#include <random>
#include <utility>
#include <functional>
using namespace std;

void Main()
{
	int N;
	cin >> N;
	set<pair<int,int>> S;
	vector<int> X(N),Y(N);
	for(int i = 0;i < N;i++)
	{
		cin >> X[i] >> Y[i];
		S.insert(make_pair(X[i],Y[i]));
	}
	map<pair<int,int>,vector<int>> mp;
	const int dx[8] = {2,2,1,-1,-2,-2,-1,1};
	const int dy[8] = {1,-1,-2,-2,-1,1,2,2};
	for(int i = 0;i < N;i++)
	{
		for(int k = 0;k < 8;k++)
		{
			int x = X[i] + dx[k];
			int y = Y[i] + dy[k];
			pair<int,int> P = make_pair(x,y);
			if(S.find(P) != S.end())
			{
				continue;
			}
			if(mp.find(P) != mp.end())
			{
				continue;
			}
			mp[P].assign(4,0);
			for(int k = 0;k < 8;k++)
			{
				int nx = x + dx[k];
				int ny = y + dy[k];
				for(int j = 0;j < N;j++)
				{
					if(nx == X[j] && ny == Y[j])
					{
						mp[P][k / 2] |= 1 << j;
					}
				}
			}
		}
	}
	vector<int> dp(1 << N,(int)1e9);
	dp[0] = 0;
	for(auto it = mp.begin();it != mp.end();it++)
	{
		vector<int> n_dp = dp;
		for(int i = 0;i < 1 << N;i++)
		{
			for(int j = 0;j < 4;j++)
			{
				int ni = i | it -> second[j];
				n_dp[ni] = min(n_dp[ni],dp[i] + 1);
			}
		}
		swap(n_dp,dp);
	}
	int ans = dp[(1 << N) - 1];
	if(ans == (int)1e9)
	{
		ans = -1;
	}
	cout << ans << "\n";
}
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int tt = 1;
	/* cin >> tt; */
	while(tt--) Main();
}