import java.util.*;
public class joi2019_ho_t4 {
static class Pair {
int x, y;
Pair(int x, int y) {
this.x = x;
this.y = y;
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof Pair)) return false;
Pair pair = (Pair) o;
return x == pair.x && y == pair.y;
}
@Override
public int hashCode() {
return Objects.hash(x, y);
}
}
static int[] dx = {-1, 0, 1, 0};
static int[] dy = {0, 1, 0, -1};
static Map<String, Integer> memo;
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int N = sc.nextInt();
List<Pair> initialCoins = new ArrayList<>();
for (int i = 0; i < 2 * N; i++) {
int x = sc.nextInt();
int y = sc.nextInt();
initialCoins.add(new Pair(x, y));
}
memo = new HashMap<>();
int result = solveDFS(N, initialCoins);
System.out.println(result == Integer.MAX_VALUE ? -1 : result);
sc.close();
}
static int solveDFS(int N, List<Pair> coins) {
String currentState = getStateString(coins);
if (memo.containsKey(currentState)) {
return memo.get(currentState);
}
if (isGoalState(coins, N)) {
return 0;
}
int minMoves = Integer.MAX_VALUE;
for (int coinIndex = 0; coinIndex < coins.size(); coinIndex++) {
Pair coin = coins.get(coinIndex);
for (int dir = 0; dir < 4; dir++) {
int newX = coin.x + dx[dir];
int newY = coin.y + dy[dir];
if (isValidMove(newX, newY, N, coins, coinIndex)) {
List<Pair> newCoins = new ArrayList<>(coins);
newCoins.set(coinIndex, new Pair(newX, newY));
int subResult = solveDFS(N, newCoins);
if (subResult != Integer.MAX_VALUE) {
minMoves = Math.min(minMoves, subResult + 1);
}
}
}
}
memo.put(currentState, minMoves);
return minMoves;
}
static boolean isValidMove(int x, int y, int N, List<Pair> coins, int excludeIndex) {
if (x < 1 || x > N || y < 1 || y > 2) return false;
for (int i = 0; i < coins.size(); i++) {
if (i == excludeIndex) continue;
if (coins.get(i).x == x && coins.get(i).y == y) return false;
}
return true;
}
static boolean isGoalState(List<Pair> coins, int N) {
Set<Pair> uniqueCoins = new HashSet<>();
for (Pair coin : coins) {
if (coin.x < 1 || coin.x > N || coin.y < 1 || coin.y > 2) {
return false;
}
if (!uniqueCoins.add(coin)) {
return false;
}
}
return true;
}
static String getStateString(List<Pair> coins) {
StringBuilder sb = new StringBuilder();
coins.sort((a, b) -> a.x == b.x ? a.y - b.y : a.x - b.x);
for (Pair coin : coins) {
sb.append(coin.x).append(',').append(coin.y).append(';');
}
return sb.toString();
}
}
