import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int N = Integer.parseInt(sc.next());
        List<Long> width = new ArrayList<>();
        for (int i = 0; i < N; i++) {
            width.add(Long.parseLong(sc.next()) + Long.parseLong(sc.next()) * 4);
        }

        width.sort((o1, o2) -> {
            if (o1 < o2) return 1;
            if (o1 > o2) return -1;
            return 0;
        });
        long max = width.get(0);

        boolean flag = true;
        int cnt = 0;
        for (int i = 1; i < N; i++) {
            if (width.get(i) == max) continue;
            if ((max - width.get(i)) % 2 == 0) {
                cnt += ((max - width.get(i)) / 2);
            } else {
                flag = false;
                break;
            }
        }
        System.out.println((flag)? cnt : "-1");
    }
}