#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic warning "-Wunused-function"
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

int dx[4][2] = {{2, 2}, {-2, -2}, {1, -1}, {1, -1}};
int dy[4][2] = {{1, -1}, {1, -1}, {2, 2}, {-2, -2}};

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  vector<P> xy(n);
  vector<P> cand;
  map<P, int> p2i;
  rep(i, n) {
    auto& [x, y] = xy[i];
    cin >> x >> y;
    p2i[{x, y}] = i;
    rep(k, 4) rep(z, 2) cand.emplace_back(x + dx[k][z], y + dy[k][z]);
  }
  constexpr int INF = 1001001001;
  VI dp(1 << n, INF);
  dp[0] = 0;
  sort(all(cand));
  cand.erase(unique(all(cand)), cand.end());
  for (auto [p, q] : cand) if (!p2i.contains(P(p, q))) {
    VI ndp = dp;
    rep(k, 4) {
      int bit = 0;
      rep(z, 2) {
        auto it = p2i.find(P(p + dx[k][z], q + dy[k][z]));
        if (it != p2i.end()) bit |= 1 << it->second;
      }
      if (!bit) continue;
      rep(s, 1 << n) if (int val = dp[s]; val != INF) {
        chmin(ndp[s | bit], dp[s] + 1);
      }
    }
    swap(dp, ndp);
  }
  int ans = dp.back();
  if (ans == INF) ans = -1;
  cout << ans << '\n';
}