#include <iostream> // cout, endl, cin
#include <string> // string, to_string, stoi
#include <vector> // vector
#include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
#include <utility> // pair, make_pair
#include <tuple> // tuple, make_tuple
#include <cstdint> // int64_t, int*_t
#include <cstdio> // printf
#include <map> // map
#include <queue> // queue, priority_queue
#include <set> // set
#include <stack> // stack
#include <deque> // deque #include <unordered_map> // unordered_map #include <unordered_set> // unordered_set #include <bitset> // bitset
#include <cctype> // isupper, islower, isdigit, toupper, tolower
#include <math.h>
#include <iomanip>
#include <functional>
#include<unordered_map>
#include <random>
#include<bitset>
#include<cassert>
using namespace std;  
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repi(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rep2(i, m, n) for (int i = (m); i < (n); ++i)
#define drep2(i, m, n) for (int i = (m)-1; i >= (n); --i)
#define drep(i, n) drep2(i, n, 0)
//#define endl '\n'
#define all(x) (x).begin(),(x).end()
#define arr(x) (x).rbegin(),(x).rend()
typedef long long ll;
typedef unsigned long long ull;
const ll inf=1e18;  
using graph = vector<vector<int> > ;
using vi=vector<int>;
using P= pair<ll,ll>;  
using vvi=vector<vi>;
using vll=vector<ll>; 
using vvll=vector<vll>;
using vp=vector<P>;
using vvp=vector<vp>;
using vd=vector<double>;
using vvd =vector<vd>;
//using vs=vector<string>;
//string T="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
//string S="abcdefghijklmnopqrstuvwxyz";
//g++ main.cpp -std=c++17 -I .  
//cout <<setprecision(20);
//cout << fixed << setprecision(10);
//cin.tie(0); ios::sync_with_stdio(false);

const double PI = acos(-1);
int vx[]={0,1,0,-1,-1,1,1,-1},vy[]={1,0,-1,0,1,1,-1,-1};
void putsYes(bool f){cout << (f?"Yes":"No") << endl;}
void putsYES(bool f){cout << (f?"YES":"NO") << endl;}
void putsFirst(bool f){cout << (f?"First":"Second") << endl;}
void debug(int test){cout << "TEST" << " " << test << endl;}
ll pow_pow(ll x,ll n,ll mod){
    if(n==0) return 1; 
    x%=mod;
    ll res=pow_pow(x*x%mod,n/2,mod);
    if(n&1)res=res*x%mod;
    return res;
}
ll gcdll(ll x,ll y){
    if(y==0)return x;
    return gcdll(y,x%y);
}
//ll INFF=1000100100100100100;
ll lcmll(ll x,ll y){
    ll X=(x/gcdll(x,y)),Y=y;
    return X*Y;
}
template<class T> bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    else return false;
}
template<class T> bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    else return false;
}
struct edge{
  int to; ll cost; 
  edge(int to,ll cost) : to(to),cost(cost){}
};
using ve=vector<edge>;
using vve=vector<ve>;
struct UnionFind { 
    vector<int> par, siz;
    UnionFind(int n) : par(n, -1) , siz(n, 1) { } 
    int root(int x) { 
        if (par[x] == -1) return x;
        else return par[x] = root(par[x]);
    }
    bool issame(int x, int y) {
        return root(x) == root(y);
    }
    bool unite(int x, int y) {
        x = root(x), y = root(y);
        if (x == y) return false; 
        if (siz[x] < siz[y]) swap(x, y);
        par[y] = x;
        siz[x] += siz[y];
        return true;
    }
    int size(int x) {
        return siz[root(x)];
    }
};
struct Compress{
   vll a;
   map<ll,ll> d,d2;
   int cnt;
   Compress() :cnt(0) {}
   void add(ll x){a.push_back(x);}
   void init(){
     set<ll>s(a.begin(),a.end());
     for(auto y:s)d[y]=cnt++;
     for(auto&y:a)y=d[y];
     for(auto u:d)d2[u.second]=u.first;
   }
   ll to(ll x){return d[x];} //変換先
   ll from(ll x){return d2[x];}//逆引き
   int size(){return cnt;}
};
vll anss;
vi dx={1,2},dy={2,1};
vi signx={1,1,-1,-1};
vi signy={-1,1,-1,1};
void solve(int test){
    int n; cin >> n;
    vi x(n),y(n);
    rep(i,n)cin >> x[i] >> y[i];
    set<P> st;
    rep(i,n){
        rep(j,2)rep(l,4){
            int nx=x[i]+dx[j]*signx[l];
            int ny=y[i]+dy[j]*signy[l];
            int ok=1;
            rep(s,n)if(x[s]==nx && y[s]==ny)ok=0;
            if(ok)st.insert(P(nx,ny));
        }
    }
    vp p(st.begin(),st.end());
    int si=p.size();
    vvi g(si,vi(4));
    auto is=[&](int x,int y,int nx,int ny,int dir){
        int d=abs(x-nx)+abs(y-ny);
        if(dir==0){
            if(x-nx==2 && abs(y-ny)==1)return 1;
            else return 0;
        }
        if(dir==1){
            if(x-nx==-2 && abs(y-ny)==1)return 1;
            else return 0;
        }
        if(dir==2){
            if(y-ny==2 && abs(x-nx)==1)return 1;
            else return 0;
        }
        if(dir==3){
            if(y-ny==-2 && abs(x-nx)==1)return 1;
            else return 0;
        }
        return 0;
    };
    rep(i,si)rep(dir,4){
        int sx=p[i].first,sy=p[i].second;
        rep(j,n){
            if(is(x[j],y[j],sx,sy,dir))g[i][dir]|=1<<j;
        }
    }
    int n2=1<<n;
    vll dp(n2,inf);
    vll old(n2,inf);
    dp[0]=0;
    rep(i,si){
        old=dp;
        rep(bit,n2)dp[bit]=inf;
        rep(bit,n2){
            ll now=old[bit];
            if(now==inf)continue;
            rep(dir,4){
                chmin(dp[bit|g[i][dir]],now+1);
            }
            chmin(dp[bit],now);
        }
    }
    ll ans=dp[n2-1];
    if(ans==inf)ans=-1;
    cout << ans <<endl;
}
//g++ cf.cpp -std=c++17 -I .
int main(){cin.tie(0);ios::sync_with_stdio(false);
    int t=1;//cin >>t;
    rep(test,t)solve(test);
    for(auto u:anss){
        cout << u << endl;
    }
}