// いろいろ高速化
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")


#ifndef HIDDEN_IN_VS // 折りたたみ用

// 警告の抑制
#define _CRT_SECURE_NO_WARNINGS

// ライブラリの読み込み
#include <bits/stdc++.h>
using namespace std;

// 型名の短縮
using ll = long long; using ull = unsigned long long; // -2^63 ~ 2^63 = 9 * 10^18(int は -2^31 ~ 2^31 = 2 * 10^9)
using pii = pair<int, int>;	using pll = pair<ll, ll>;	using pil = pair<int, ll>;	using pli = pair<ll, int>;
using vi = vector<int>;		using vvi = vector<vi>;		using vvvi = vector<vvi>;	using vvvvi = vector<vvvi>;
using vl = vector<ll>;		using vvl = vector<vl>;		using vvvl = vector<vvl>;	using vvvvl = vector<vvvl>;
using vb = vector<bool>;	using vvb = vector<vb>;		using vvvb = vector<vvb>;
using vc = vector<char>;	using vvc = vector<vc>;		using vvvc = vector<vvc>;
using vd = vector<double>;	using vvd = vector<vd>;		using vvvd = vector<vvd>;
template <class T> using priority_queue_rev = priority_queue<T, vector<T>, greater<T>>;
using Graph = vvi;

// 定数の定義
const double PI = acos(-1);
const vi DX = { 1, 0, -1, 0 }; // 4 近傍(下,右,上,左)
const vi DY = { 0, 1, 0, -1 };
int INF = 1001001001; ll INFL = 4004004003104004004LL; // (int)INFL = 1010931620;

// 入出力高速化
struct fast_io { fast_io() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(18); } } fastIOtmp;

// 汎用マクロの定義
#define all(a) (a).begin(), (a).end()
#define sz(x) ((int)(x).size())
#define lbpos(a, x) (int)distance((a).begin(), std::lower_bound(all(a), x))
#define ubpos(a, x) (int)distance((a).begin(), std::upper_bound(all(a), x))
#define Yes(b) {cout << ((b) ? "Yes\n" : "No\n");}
#define rep(i, n) for(int i = 0, i##_len = int(n); i < i##_len; ++i) // 0 から n-1 まで昇順
#define repi(i, s, t) for(int i = int(s), i##_end = int(t); i <= i##_end; ++i) // s から t まで昇順
#define repir(i, s, t) for(int i = int(s), i##_end = int(t); i >= i##_end; --i) // s から t まで降順
#define repe(v, a) for(const auto& v : (a)) // a の全要素(変更不可能)
#define repea(v, a) for(auto& v : (a)) // a の全要素(変更可能)
#define repb(set, d) for(int set = 0, set##_ub = 1 << int(d); set < set##_ub; ++set) // d ビット全探索(昇順)
#define repis(i, set) for(int i = lsb(set), bset##i = set; i >= 0; bset##i -= 1 << i, i = lsb(bset##i)) // set の全要素(昇順)
#define repp(a) sort(all(a)); for(bool a##_perm = true; a##_perm; a##_perm = next_permutation(all(a))) // a の順列全て(昇順)
#define smod(n, m) ((((n) % (m)) + (m)) % (m)) // 非負mod
#define uniq(a) {sort(all(a)); (a).erase(unique(all(a)), (a).end());} // 重複除去
#define EXIT(a) {cout << (a) << endl; exit(0);} // 強制終了
#define inQ(x, y, u, l, d, r) ((u) <= (x) && (l) <= (y) && (x) < (d) && (y) < (r)) // 半開矩形内判定

// 汎用関数の定義
template <class T> inline ll powi(T n, int k) { ll v = 1; rep(i, k) v *= n; return v; }
template <class T> inline bool chmax(T& M, const T& x) { if (M < x) { M = x; return true; } return false; } // 最大値を更新(更新されたら true を返す)
template <class T> inline bool chmin(T& m, const T& x) { if (m > x) { m = x; return true; } return false; } // 最小値を更新(更新されたら true を返す)
template <class T> inline T get(T set, int i) { return (set >> i) & T(1); }

// 演算子オーバーロード
template <class T, class U> inline istream& operator>>(istream& is, pair<T, U>& p) { is >> p.first >> p.second; return is; }
template <class T> inline istream& operator>>(istream& is, vector<T>& v) { repea(x, v) is >> x; return is; }
template <class T> inline vector<T>& operator--(vector<T>& v) { repea(x, v) --x; return v; }
template <class T> inline vector<T>& operator++(vector<T>& v) { repea(x, v) ++x; return v; }

#endif // 折りたたみ用


#if __has_include(<atcoder/all>)
#include <atcoder/all>
using namespace atcoder;

#ifdef _MSC_VER
#include "localACL.hpp"
#endif

//using mint = modint1000000007;
using mint = modint998244353;
//using mint = modint; // mint::set_mod(m);

namespace atcoder {
	inline istream& operator>>(istream& is, mint& x) { ll x_; is >> x_; x = x_; return is; }
	inline ostream& operator<<(ostream& os, const mint& x) { os << x.val(); return os; }
}
using vm = vector<mint>; using vvm = vector<vm>; using vvvm = vector<vvm>; using vvvvm = vector<vvvm>; using pim = pair<int, mint>;
#endif


#ifdef _MSC_VER // 手元環境(Visual Studio)
#include "local.hpp"
#else // 提出用(gcc)
inline int popcount(int n) { return __builtin_popcount(n); }
inline int popcount(ll n) { return __builtin_popcountll(n); }
inline int lsb(int n) { return n != 0 ? __builtin_ctz(n) : -1; }
inline int lsb(ll n) { return n != 0 ? __builtin_ctzll(n) : -1; }
inline int msb(int n) { return n != 0 ? (31 - __builtin_clz(n)) : -1; }
inline int msb(ll n) { return n != 0 ? (63 - __builtin_clzll(n)) : -1; }
#define dump(...)
#define dumpel(v)
#define dump_list(v)
#define dump_mat(v)
#define input_from_file(f)
#define output_to_file(f)
#define Assert(b) { if (!(b)) while (1) cout << "OLE"; }
#endif


void WA() {
	auto start = chrono::system_clock::now();

	int n;
	cin >> n;

	vi x(n), y(n);
	rep(i, n) cin >> x[i] >> y[i];

	++x; ++x;
	++y; ++y;

	vvi ex(2010, vi(2010));
	rep(i, n) ex[x[i]][y[i]] = 1;

	const vi dx = { 2, 2, -2, -2, -1, 1, -1, 1 };
	const vi dy = { -1, 1, -1, 1, -2, -2, 2, 2 };

	int res = INF;

	mt19937_64 mt((int)time(NULL));
	uniform_int_distribution<int> rnd(0, 7);

	while (1) {
		unordered_map<ll, int> d;

		bool ok = true;
		rep(i, n) {
			int k = rnd(mt);

			if (ex[x[i] + dx[k]][y[i] + dy[k]]) {
				ok = false;
				break;
			}

			ll h = (x[i] + dx[k]) * (ll)INF + (y[i] + dy[k]);
			int dir = k / 2;

			if (d.count(h) && d[h] != dir) {
				ok = false;
				break;
			}
			d[h] = dir;
		}

		if (ok) chmin(res, sz(d));

		auto now = chrono::system_clock::now();
		auto msec = chrono::duration_cast<chrono::milliseconds>(now - start).count();
		if (msec >= 1950) break;
	}

	if (res == INF) res = -1;

	cout << res << endl;
}


int main() {
//	input_from_file("input.txt");
//	output_to_file("output.txt");
		
	auto start = chrono::system_clock::now();

	int n;
	cin >> n;

	vi x(n), y(n);
	rep(i, n) cin >> x[i] >> y[i];

	vector<pii> xy(n);
	rep(i, n) xy[i] = { x[i], y[i] };
	sort(all(xy));

	rep(i, n) tie(x[i], y[i]) = xy[i];

	rep(i, n) {
		x[i] += 4;
		y[i] += 4;
	}

	constexpr int W = 1010;
	int ex[W][W];
	rep(i, W) rep(j, W) ex[i][j] = -2;
	rep(i, n) ex[x[i]][y[i]] = -1;

	constexpr int dx[8] = {2, 2, -2, -2, -1, 1, -1, 1};
	constexpr int dy[8] = {-1, 1, -1, 1, -2, -2, 2, 2};

	int deg[W][W];
	rep(i, W) rep(j, W) deg[i][j] = 0;

	rep(i, n) rep(k, 8) {
		int x2 = x[i] + dx[k];
		int y2 = y[i] + dy[k];

		deg[x2][y2]++;
	}

	int res = INF;

	int cost = 0;
	function<void(int)> rf = [&](int i) {
		// もう時間がないので強制終了.
		auto now = chrono::system_clock::now();
		auto msec = chrono::duration_cast<chrono::milliseconds>(now - start).count();
		if (msec >= 1980) {
			if (res == INF) res = -1;
			EXIT(res);
		}

		// これ以上調べても無駄なので打ち切り.
		if (cost >= res) return;

		if (i == n) {
			chmin(res, cost);
			return;
		}

		int lx = -1, ly = -1, lk = -1;

		rep(k, 8) {
			int x2 = x[i] + dx[k];
			int y2 = y[i] + dy[k];
			if (ex[x2][y2] == -1) continue;
			if (ex[x2][y2] != -2 && ex[x2][y2] != k / 2) continue;

			// 葉は全部同一視できるので調べるのはどれか 1 つでいい.
			if (deg[x2][y2] == 1) {
				if (lx == -1) {
					lx = x2;
					ly = y2;
					lk = k;
				}
			}
			else {
				int ex_prv = ex[x2][y2];

				if (ex[x2][y2] == -2) cost++;
				ex[x2][y2] = k / 2;

				rf(i + 1);

				if (ex_prv == -2) cost--;
				ex[x2][y2] = ex_prv;
			}
		}

		if (lx != -1) {
			int ex_prv = ex[lx][ly];
			
			if (ex[lx][ly] == -2) cost++;
			ex[lx][ly] = lk / 2;

			rf(i + 1);

			if (ex_prv == -2) cost--;
			ex[lx][ly] = ex_prv;
		}
	};
	rf(0);

	if (res == INF) res = -1;

	cout << res << endl;
}