#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 1000000000000000001

int main(){
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	
	int n;
	cin>>n;
	
	vector<int> x(n),y(n);
	vector<pair<int,int>> t;
	rep(i,n){
		cin>>x[i]>>y[i];
		
		for(int j=-2;j<=2;j++){
			for(int k=-2;k<=2;k++){
				if(k==0||j==0)continue;
				if(abs(j)+abs(k)!=3)continue;
				t.emplace_back(x[i]+j,y[i]+k);
			}
		}
	}
	sort(t.begin(),t.end());
	t.erase(unique(t.begin(),t.end()),t.end());
	
	vector<int> dp(1<<n,Inf32);
	dp[0] = 0;
	rep(i,t.size()){
		vector<int> ndp = dp;
		bool f = true;
		rep(j,n){
			if(x[j]==t[i].first && y[j]==t[i].second)f = false;
		}
		if(!f)continue;
		{
			int mask = 0;
			rep(j,n){
				if(x[j]==t[i].first + 2 && y[j] == t[i].second + 1)mask |= 1<<j;
				if(x[j]==t[i].first + 2 && y[j] == t[i].second - 1)mask |= 1<<j;
			}
			rep(j,1<<n){
				if(dp[j]==Inf32)continue;
				ndp[j|mask] = min(ndp[j|mask],dp[j] + 1);
			}
		}
		{
			int mask = 0;
			rep(j,n){
				if(x[j]==t[i].first - 2 && y[j] == t[i].second + 1)mask |= 1<<j;
				if(x[j]==t[i].first - 2 && y[j] == t[i].second - 1)mask |= 1<<j;
			}
			rep(j,1<<n){
				if(dp[j]==Inf32)continue;
				ndp[j|mask] = min(ndp[j|mask],dp[j] + 1);
			}
		}
		{
			int mask = 0;
			rep(j,n){
				if(x[j]==t[i].first + 1 && x[j] == t[i].second + 2)mask |= 1<<j;
				if(x[j]==t[i].first - 1 && x[j] == t[i].second + 2)mask |= 1<<j;
			}
			rep(j,1<<n){
				if(dp[j]==Inf32)continue;
				ndp[j|mask] = min(ndp[j|mask],dp[j] + 1);
			}
		}
		{
			int mask = 0;
			rep(j,n){
				if(x[j]==t[i].first + 1 && x[j] == t[i].second - 2)mask |= 1<<j;
				if(x[j]==t[i].first - 1 && x[j] == t[i].second - 2)mask |= 1<<j;
			}
			rep(j,1<<n){
				if(dp[j]==Inf32)continue;
				ndp[j|mask] = min(ndp[j|mask],dp[j] + 1);
			}
		}
		swap(dp,ndp);
	}
	int ans = dp.back();
	if(ans==Inf32)ans = -1;
	cout<<ans<<endl;
	
	return 0;
}