// writer

#include <bits/stdc++.h>
using namespace std;

int main() {
    int N;
    cin >> N;

    vector<int> x(N), y(N);

    // その座標にいる UMA の集合を bit で持っておく。
    int uma_set[1100][1100] = {};
    for (int i = 0; i < N; i++) {
        cin >> x[i] >> y[i];
        // 負になってバグるのを防ぐため +5 ずつしておく
        x[i] += 5;
        y[i] += 5;
        uma_set[x[i]][y[i]] = (1 << i);
    }

    // 設置する点の候補を列挙
    set<pair<int, int>> candidates;
    const int dx[] = {2, 1, -1, -2, -2, -1, 1, 2};
    const int dy[] = {1, 2, 2, 1, -1, -2, -2, -1};
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < 8; j++) {
            candidates.insert({x[i] + dx[j], y[i] + dy[j]});
        }
    }
    // UMA のいる場所を除く
    for (int i = 0; i < N; i++) {
        pair<int, int> p = {x[i], y[i]};
        if (candidates.count(p)) candidates.erase(p);
    }

    // dp
    vector<int> dp(1 << N, numeric_limits<int>::max());
    dp[0] = 0;
    for (auto &&[a, b] : candidates) {
        auto new_dp = dp;

        for (int bits = 0; bits < (1 << N); bits++) {
            if (dp[bits] == numeric_limits<int>::max()) continue;

            const int dx1[] = {-1, -1, +2, -2};
            const int dy1[] = {+2, -2, -1, -1};
            const int dx2[] = {+1, +1, +2, -2};
            const int dy2[] = {+2, -2, +1, +1};

            for (int dir = 0; dir < 4; dir++) {
                int bits_next = bits;

                // 向いた先の UMA を bits_next に加える
                bits_next |= uma_set[a + dx1[dir]][b + dy1[dir]];
                bits_next |= uma_set[a + dx2[dir]][b + dy2[dir]];

                new_dp[bits_next] = min(new_dp[bits_next], dp[bits] + 1);
            }
        }

        dp.swap(new_dp);
    }

    cout << (dp.back() != numeric_limits<int>::max() ? dp.back() : -1) << endl;
}