#include <atcoder/all>
#include <bits/stdc++.h>
using ll = long long;
using ull = unsigned long long;
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define REP(i, m, n) for(int i = (int)(m); i < (int)(n); i++)
using namespace std;
using namespace atcoder;
using mint = modint998244353;
const int inf = 1000000007;
const ll longinf = 1ll << 60;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int n;
    cin >> n;
    vector a(1001, vector(1001, 0));
    rep(i, n) {
        int x, y;
        cin >> x >> y;
        a[x][y] = 1 << i;
    }
    int all = 1 << n;
    vector<int> dp(all, inf);
    dp[0] = 0;
    auto get = [&](int x, int y) {
        if(0 <= x && x <= 1000 && 0 <= y && y <= 1000) {
            return a[x][y];
        } else {
            return 0;
        }
    };
    REP(x, -2, 1003)
    REP(y, -2, 1003) {
        if(get(x, y) > 0) {
            continue;
        }
        vector<int> vals;
        {
            int u = get(x + 2, y + 1) | get(x + 2, y - 1);
            if(u > 0)
                vals.push_back(u);
        }
        {
            int u = get(x - 2, y + 1) | get(x - 2, y - 1);
            if(u > 0)
                vals.push_back(u);
        }
        {
            int u = get(x + 1, y + 2) | get(x - 1, y + 2);
            if(u > 0)
                vals.push_back(u);
        }
        {
            int u = get(x + 1, y - 2) | get(x - 1, y - 2);
            if(u > 0)
                vals.push_back(u);
        }
        if(vals.size() > 0) {
            vector<int> ndp = dp;
            rep(i, all) for(auto v : vals) {
                ndp[i | v] = min(ndp[i | v], dp[i] + 1);
            }
            dp.swap(ndp);
        }
    }
    if(dp[all - 1] == inf) {
        cout << -1 << endl;
    } else {
        cout << dp[all - 1] << endl;
    }
    return 0;
}