#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using mint = modint998244353;

//多倍長整数//
//#include <boost/multiprecision/cpp_int.hpp>
//namespace mp = boost::multiprecision;
//using bint = mp::cpp_int;

const int INF = 2e9;
const int MOD = 998244353;
const long long LINF = 5e18;

using ll = long long;
using vi = vector<int>;
using vl = vector<long long>;
using vs = vector<string>;
using vc = vector<char>;
using vb = vector<bool>;
using vvi = vector<vector<int>>;
using vvvi = vector<vector<vector<int>>>;
using vvvvi = vector<vector<vector<vector<int>>>>;
using vvl = vector<vector<long long>>;
using vvvl = vector<vector<vector<long long>>>;
using vvvvl = vector<vector<vector<vector<long long>>>>;
using vvc = vector<vector<char>>;
using vvb = vector<vector<bool>>;
using vvvb = vector<vector<vector<bool>>>;
using vvvvb = vector<vector<vector<vector<bool>>>>;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define dump(x)  cout << #x << " = " << (x) << endl;
#define Yes(n) cout << ((n) ? "Yes" : "No"  ) << endl
#define ALL(obj) (obj).begin(),(obj).end()

int main(){
	int n;
	cin >> n;
	vi x(n),y(n);
	rep(i,n) cin >> x[i] >> y[i];
	vector<int> dp(1<<n,INF);
	int dx[8] = {2,2,1,-1,-2,-2,-1,1};
	int dy[8] = {-1,1,2,2,1,-1,-2,-2};
	dp[0] = 0;
	rep(i,1<<n){
		rep(j,n){
			if(i & (1<<j)) continue;
			rep(k,8){
				bool ok = true;
				rep(l,n){
					if(j == l) continue;
					if(i & (1<<l)) continue;
					if(x[j] + dx[k] == x[l] && y[j] + dy[k] == y[l]) ok = false;
					rep(m,8) if(x[j] + dx[k] + dx[m] == x[l] && y[j] + dy[k] + dy[m] == y[l]) ok = false;
				}
				if(ok){
					dp[i | (1<<j)] = min(dp[i | (1<<j)],dp[i] + 1);
					break;
				}
			}
		}
		for(int j = 0;j < n - 1;j++){
			if(i & (1<<j)) continue;
			for(int k = j + 1;k < n;k++){
				if(i & (1<<k)) continue;
				if(x[j] == x[k] && y[j] + 2 == y[k]){
					int nx = x[j] + 2,ny = y[j] + 1;
					bool ok = true;
					rep(l,n){
						if(j == l) continue;
						if(k == l) continue;
						if(i & (1<<l)) continue;
						if(nx == x[l] && ny == y[l]) ok = false;
						rep(m,8) if(nx + dx[m] == x[l] && ny + dy[m] == y[l]) ok = false;
					}
					if(ok){
						dp[i | (1<<j) | (1<<k)] = min(dp[i | (1<<j) | (1<<k)],dp[i] + 1);
					}

					nx = x[j] - 2;
					ny = y[j] + 1;
					ok = true;
					rep(l,n){
						if(j == l) continue;
						if(k == l) continue;
						if(i & (1<<l)) continue;
						if(nx == x[l] && ny == y[l]) ok = false;
						rep(m,8) if(nx + dx[m] == x[l] && ny + dy[m] == y[l]) ok = false;
					}
					if(ok){
						dp[i | (1<<j) | (1<<k)] = min(dp[i | (1<<j) | (1<<k)],dp[i] + 1);
					}
				}
				if(x[j] + 2 == x[k] && y[j] == y[k]){
					int nx = x[j] + 1,ny = y[j] + 2;
					bool ok = true;
					rep(l,n){
						if(j == l) continue;
						if(k == l) continue;
						if(i & (1<<l)) continue;
						if(nx == x[l] && ny == y[l]) ok = false;
						rep(m,8) if(nx + dx[m] == x[l] && ny + dy[m] == y[l]) ok = false;
					}
					if(ok){
						dp[i | (1<<j) | (1<<k)] = min(dp[i | (1<<j) | (1<<k)],dp[i] + 1);
					}

					nx = x[j] + 1;
					ny = y[j] - 2;
					ok = true;
					rep(l,n){
						if(j == l) continue;
						if(k == l) continue;
						if(i & (1<<l)) continue;
						if(nx == x[l] && ny == y[l]) ok = false;
						rep(m,8) if(nx + dx[m] == x[l] && ny + dy[m] == y[l]) ok = false;
					}
					if(ok){
						dp[i | (1<<j) | (1<<k)] = min(dp[i | (1<<j) | (1<<k)],dp[i] + 1);
					}
				}

				if(x[j] == x[k] && y[j] - 2 == y[k]){
					int nx = x[j] + 2,ny = y[j] - 1;
					bool ok = true;
					rep(l,n){
						if(j == l) continue;
						if(k == l) continue;
						if(i & (1<<l)) continue;
						if(nx == x[l] && ny == y[l]) ok = false;
						rep(m,8) if(nx + dx[m] == x[l] && ny + dy[m] == y[l]) ok = false;
					}
					if(ok){
						dp[i | (1<<j) | (1<<k)] = min(dp[i | (1<<j) | (1<<k)],dp[i] + 1);
					}

					nx = x[j] - 2;
					ny = y[j] - 1;
					ok = true;
					rep(l,n){
						if(j == l) continue;
						if(k == l) continue;
						if(i & (1<<l)) continue;
						if(nx == x[l] && ny == y[l]) ok = false;
						rep(m,8) if(nx + dx[m] == x[l] && ny + dy[m] == y[l]) ok = false;
					}
					if(ok){
						dp[i | (1<<j) | (1<<k)] = min(dp[i | (1<<j) | (1<<k)],dp[i] + 1);
					}
				}

				if(x[j] - 2 == x[k] && y[j] == y[k]){
					int nx = x[j] - 1,ny = y[j] + 2;
					bool ok = true;
					rep(l,n){
						if(j == l) continue;
						if(k == l) continue;
						if(i & (1<<l)) continue;
						if(nx == x[l] && ny == y[l]) ok = false;
						rep(m,8) if(nx + dx[m] == x[l] && ny + dy[m] == y[l]) ok = false;
					}
					if(ok){
						dp[i | (1<<j) | (1<<k)] = min(dp[i | (1<<j) | (1<<k)],dp[i] + 1);
					}

					nx = x[j] - 1;
					ny = y[j] - 2;
					ok = true;
					rep(l,n){
						if(j == l) continue;
						if(k == l) continue;
						if(i & (1<<l)) continue;
						if(nx == x[l] && ny == y[l]) ok = false;
						rep(m,8) if(nx + dx[m] == x[l] && ny + dy[m] == y[l]) ok = false;
					}
					if(ok){
						dp[i | (1<<j) | (1<<k)] = min(dp[i | (1<<j) | (1<<k)],dp[i] + 1);
					}
				}
			}
		}
	}
	if(dp[(1<<n) - 1] == INF) cout << -1 << endl;
	else cout << dp[(1<<n) - 1] << endl;
	return 0;
}