#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=300005,INF=1<<30;

// フローのみ

// from: https://gist.github.com/yosupo06/ddd51afb727600fd95d9d8ad6c3c80c9
// (based on AtCoder STL)

#ifndef ATCODER_INTERNAL_QUEUE_HPP
#define ATCODER_INTERNAL_QUEUE_HPP 1
#include <vector>
namespace atcoder {
    namespace internal {
        template <class T> struct simple_queue {
            std::vector<T> payload;
            int pos = 0;
            void reserve(int n) { payload.reserve(n); }
            int size() const { return int(payload.size()) - pos; }
            bool empty() const { return pos == int(payload.size()); }
            void push(const T& t) { payload.push_back(t); }
            T& front() { return payload[pos]; }
            void clear() {
                payload.clear();
                pos = 0;
            }
            void pop() { pos++; }
        };
    }  // namespace internal
}  // namespace atcoder
#endif  // ATCODER_INTERNAL_QUEUE_HPP

#ifndef ATCODER_MAXFLOW_HPP
#define ATCODER_MAXFLOW_HPP 1
#include <algorithm>
#include <cassert>
#include <limits>
#include <queue>
#include <vector>
namespace atcoder {
    template <class Cap> struct mf_graph {
    public:
        mf_graph() : _n(0) {}
        mf_graph(int n) : _n(n), g(n) {}
        int add_edge(int from, int to, Cap cap) {
            assert(0 <= from && from < _n);
            assert(0 <= to && to < _n);
            assert(0 <= cap);
            int m = int(pos.size());
            pos.push_back({from, int(g[from].size())});
            g[from].push_back(_edge{to, int(g[to].size()), cap});
            g[to].push_back(_edge{from, int(g[from].size()) - 1, 0});
            return m;
        }
        struct edge {
            int from, to;
            Cap cap, flow;
        };
        edge get_edge(int i) {
            int m = int(pos.size());
            assert(0 <= i && i < m);
            auto _e = g[pos[i].first][pos[i].second];
            auto _re = g[_e.to][_e.rev];
            return edge{pos[i].first, _e.to, _e.cap + _re.cap, _re.cap};
        }
        std::vector<edge> edges() {
            int m = int(pos.size());
            std::vector<edge> result;
            for (int i = 0; i < m; i++) {
                result.push_back(get_edge(i));
            }
            return result;
        }
        void change_edge(int i, Cap new_cap, Cap new_flow) {
            int m = int(pos.size());
            assert(0 <= i && i < m);
            assert(0 <= new_flow && new_flow <= new_cap);
            auto& _e = g[pos[i].first][pos[i].second];
            auto& _re = g[_e.to][_e.rev];
            _e.cap = new_cap - new_flow;
            _re.cap = new_flow;
        }
        Cap flow(int s, int t) {
            return flow(s, t, std::numeric_limits<Cap>::max());
        }
        Cap flow(int s, int t, Cap flow_limit) {
            assert(0 <= s && s < _n);
            assert(0 <= t && t < _n);
            std::vector<int> level(_n), iter(_n);
            internal::simple_queue<int> que;
            auto bfs = [&]() {
                std::fill(level.begin(), level.end(), -1);
                level[s] = 0;
                que.clear();
                que.push(s);
                while (!que.empty()) {
                    int v = que.front();
                    que.pop();
                    for (auto e : g[v]) {
                        if (e.cap == 0 || level[e.to] >= 0) continue;
                        level[e.to] = level[v] + 1;
                        if (e.to == t) return;
                        que.push(e.to);
                    }
                }
            };
            auto dfs = [&](auto self, int v, Cap up) {
                if (v == s) return up;
                Cap res = 0;
                int level_v = level[v];
                for (int& i = iter[v]; i < int(g[v].size()); i++) {
                    _edge& e = g[v][i];
                    if (level_v <= level[e.to] || g[e.to][e.rev].cap == 0) continue;
                    Cap d =
                    self(self, e.to, std::min(up - res, g[e.to][e.rev].cap));
                    if (d <= 0) continue;
                    g[v][i].cap += d;
                    g[e.to][e.rev].cap -= d;
                    res += d;
                    if (res == up) break;
                }
                return res;
            };
            Cap flow = 0;
            while (flow < flow_limit) {
                bfs();
                if (level[t] == -1) break;
                std::fill(iter.begin(), iter.end(), 0);
                while (flow < flow_limit) {
                    Cap f = dfs(dfs, t, flow_limit - flow);
                    if (!f) break;
                    flow += f;
                }
            }
            return flow;
        }
        std::vector<bool> min_cut(int s) {
            std::vector<bool> visited(_n);
            internal::simple_queue<int> que;
            que.push(s);
            while (!que.empty()) {
                int p = que.front();
                que.pop();
                visited[p] = true;
                for (auto e : g[p]) {
                    if (e.cap && !visited[e.to]) {
                        visited[e.to] = true;
                        que.push(e.to);
                    }
                }
            }
            return visited;
        }
    private:
        int _n;
        struct _edge {
            int to, rev;
            Cap cap;
        };
        std::vector<std::pair<int, int>> pos;
        std::vector<std::vector<_edge>> g;
    };
}  // namespace atcoder
#endif  // ATCODER_MAXFLOW_HPP
#ifndef ATCODER_MINCOSTFLOW_HPP
#define ATCODER_MINCOSTFLOW_HPP 1
#include <algorithm>
#include <cassert>
#include <limits>
#include <queue>
#include <vector>
namespace atcoder {
    template <class Cap, class Cost> struct mcf_graph {
    public:
        mcf_graph() {}
        mcf_graph(int n) : _n(n), g(n) {}
        int add_edge(int from, int to, Cap cap, Cost cost) {
            assert(0 <= from && from < _n);
            assert(0 <= to && to < _n);
            int m = int(pos.size());
            pos.push_back({from, int(g[from].size())});
            g[from].push_back(_edge{to, int(g[to].size()), cap, cost});
            g[to].push_back(_edge{from, int(g[from].size()) - 1, 0, -cost});
            return m;
        }
        struct edge {
            int from, to;
            Cap cap, flow;
            Cost cost;
        };
        edge get_edge(int i) {
            int m = int(pos.size());
            assert(0 <= i && i < m);
            auto _e = g[pos[i].first][pos[i].second];
            auto _re = g[_e.to][_e.rev];
            return edge{
                pos[i].first, _e.to, _e.cap + _re.cap, _re.cap, _e.cost,
            };
        }
        std::vector<edge> edges() {
            int m = int(pos.size());
            std::vector<edge> result(m);
            for (int i = 0; i < m; i++) {
                result[i] = get_edge(i);
            }
            return result;
        }
        std::pair<Cap, Cost> flow(int s, int t) {
            return flow(s, t, std::numeric_limits<Cap>::max());
        }
        std::pair<Cap, Cost> flow(int s, int t, Cap flow_limit) {
            return slope(s, t, flow_limit).back();
        }
        std::vector<std::pair<Cap, Cost>> slope(int s, int t) {
            return slope(s, t, std::numeric_limits<Cap>::max());
        }
        std::vector<std::pair<Cap, Cost>> slope(int s, int t, Cap flow_limit) {
            assert(0 <= s && s < _n);
            assert(0 <= t && t < _n);
            assert(s != t);
            // variants (C = maxcost):
            // -(n-1)C <= dual[s] <= dual[i] <= dual[t] = 0
            // reduced cost (= e.cost + dual[e.from] - dual[e.to]) >= 0 for all edge
            std::vector<Cost> dual(_n, 0), dist(_n);
            std::vector<int> pv(_n), pe(_n);
            std::vector<bool> vis(_n);
            auto dual_ref = [&]() {
                std::fill(dist.begin(), dist.end(),
                          std::numeric_limits<Cost>::max());
                std::fill(pv.begin(), pv.end(), -1);
                std::fill(pe.begin(), pe.end(), -1);
                std::fill(vis.begin(), vis.end(), false);
                struct Q {
                    Cost key;
                    int to;
                    bool operator<(Q r) const { return key > r.key; }
                };
                std::priority_queue<Q> que;
                dist[s] = 0;
                que.push(Q{0, s});
                while (!que.empty()) {
                    int v = que.top().to;
                    que.pop();
                    if (vis[v]) continue;
                    vis[v] = true;
                    if (v == t) break;
                    // dist[v] = shortest(s, v) + dual[s] - dual[v]
                    // dist[v] >= 0 (all reduced cost are positive)
                    // dist[v] <= (n-1)C
                    for (int i = 0; i < int(g[v].size()); i++) {
                        auto e = g[v][i];
                        if (vis[e.to] || !e.cap) continue;
                        // |-dual[e.to] + dual[v]| <= (n-1)C
                        // cost <= C - -(n-1)C + 0 = nC
                        Cost cost = e.cost - dual[e.to] + dual[v];
                        if (dist[e.to] - dist[v] > cost) {
                            dist[e.to] = dist[v] + cost;
                            pv[e.to] = v;
                            pe[e.to] = i;
                            que.push(Q{dist[e.to], e.to});
                        }
                    }
                }
                if (!vis[t]) {
                    return false;
                }
                for (int v = 0; v < _n; v++) {
                    if (!vis[v]) continue;
                    // dual[v] = dual[v] - dist[t] + dist[v]
                    //         = dual[v] - (shortest(s, t) + dual[s] - dual[t]) + (shortest(s, v) + dual[s] - dual[v])
                    //         = - shortest(s, t) + dual[t] + shortest(s, v)
                    //         = shortest(s, v) - shortest(s, t) >= 0 - (n-1)C
                    dual[v] -= dist[t] - dist[v];
                }
                return true;
            };
            Cap flow = 0;
            Cost cost = 0, prev_cost = -1;
            std::vector<std::pair<Cap, Cost>> result;
            result.push_back({flow, cost});
            while (flow < flow_limit) {
                if (!dual_ref()) break;
                Cap c = flow_limit - flow;
                for (int v = t; v != s; v = pv[v]) {
                    c = std::min(c, g[pv[v]][pe[v]].cap);
                }
                for (int v = t; v != s; v = pv[v]) {
                    auto& e = g[pv[v]][pe[v]];
                    e.cap -= c;
                    g[v][e.rev].cap += c;
                }
                Cost d = -dual[s];
                flow += c;
                cost += c * d;
                if (prev_cost == d) {
                    result.pop_back();
                }
                result.push_back({flow, cost});
                prev_cost = d;
            }
            return result;
        }
    private:
        int _n;
        struct _edge {
            int to, rev;
            Cap cap;
            Cost cost;
        };
        std::vector<std::pair<int, int>> pos;
        std::vector<std::vector<_edge>> g;
    };
}  // namespace atcoder
#endif  // ATCODER_MINCOSTFLOW_HPP

bool NG[1100][1100];

vector<int> dh={-2,-2,-1,1,2,2,1,-1},dw={-1,1,2,2,1,-1,-2,-2};

int ans=INF;

vector<pair<int,int>> S,T;

bool seen[18];

void solve(int u){
    if(u==si(S)){
        int V=si(S)-si(T);
        vector<pair<int,int>> use;
        for(int i=0;i<si(S);i++){
            if(seen[i]) continue;
            for(int k=0;k<8;k++){
                int toh=S[i].fi+dh[k],tow=S[i].se+dw[k];
                if(NG[toh][tow]) continue;
                use.push_back(mp(toh,tow));
            }
        }
        for(int i=0;i<si(T);i++){
            auto [x,y]=T[i];
            auto [a,b]=S[x];
            auto [c,d]=S[y];
            
            int h=(a+c)/2,w=(b+d)/2;
            
            if(a==c){
                if(!NG[h-2][w]) use.push_back(mp(h-2,w));
                if(!NG[h+2][w]) use.push_back(mp(h+2,w));
            }
            if(b==d){
                if(!NG[h][w-2]) use.push_back(mp(h,w-2));
                if(!NG[h][w+2]) use.push_back(mp(h,w+2));
            }
        }
        
        sort(all(use));
        use.erase(unique(all(use)),use.end());
        
        auto id=[&](int a,int b){
            return lower_bound(all(use),mp(a,b))-use.begin();
        };
        
        atcoder::mf_graph<int> G(V+si(use)+2);
        int s=V+si(use),t=s+1;
        
        int ii=0;
        for(int i=0;i<si(S);i++){
            if(seen[i]) continue;
            for(int k=0;k<8;k++){
                int toh=S[i].fi+dh[k],tow=S[i].se+dw[k];
                if(NG[toh][tow]) continue;
                G.add_edge(ii,V+id(toh,tow),1);
                //use.push_back(mp(toh,tow));
            }
            ii++;
        }
        for(int i=0;i<si(T);i++){
            auto [x,y]=T[i];
            auto [a,b]=S[x];
            auto [c,d]=S[y];
            
            int h=(a+c)/2,w=(b+d)/2;
            
            if(a==c){
                if(!NG[h-2][w]) G.add_edge(si(S)-si(T)*2+i,V+id(h-2,w),1);
                if(!NG[h+2][w]) G.add_edge(si(S)-si(T)*2+i,V+id(h+2,w),1);
            }
            if(b==d){
                if(!NG[h][w-2]) G.add_edge(si(S)-si(T)*2+i,V+id(h,w-2),1);
                if(!NG[h][w+2]) G.add_edge(si(S)-si(T)*2+i,V+id(h,w+2),1);
            }
        }
        
        for(int i=0;i<V;i++) G.add_edge(s,i,1);
        for(int i=0;i<si(use);i++) G.add_edge(V+i,t,1);
        
        if(G.flow(s,t)==V) chmin(ans,V);
        
        return;
    }
    
    if(seen[u]) solve(u+1);
    else{
        solve(u+1);
        for(int x=u+1;x<si(S);x++){
            if(seen[x]) continue;
            
            auto [a,b]=S[u];
            auto [c,d]=S[x];
            if(a==c&&abs(b-d)==2){
                T.push_back(mp(u,x));
                seen[u]=seen[x]=true;
                solve(u+1);
                seen[u]=seen[x]=false;
                T.pop_back();
            }else if(b==d&&abs(a-c)==2){
                T.push_back(mp(u,x));
                seen[u]=seen[x]=true;
                solve(u+1);
                seen[u]=seen[x]=false;
                T.pop_back();
            }else{
                
            }
        }
    }
}

int main(){
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int N;cin>>N;
    for(int i=0;i<N;i++){
        int x,y;cin>>x>>y;
        x+=10;
        y+=10;
        NG[x][y]=true;
        S.push_back(mp(x,y));
    }
    
    solve(0);
    
    if(ans==INF) cout<<-1<<endl;
    else cout<<ans<<endl;
}