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());

        long X = Long.parseLong(sc.next());
        long Y = Long.parseLong(sc.next());

        long num = 0;

        boolean flag = false;
        if (Y - X > 0) {
            num = Y - X;
            for (int i = 0; i < N - 1; i++) {
                X = Long.parseLong(sc.next());
                Y = Long.parseLong(sc.next());

                if (Y - X > 0) {
                    if (!(num == Y - X)) {
                        flag = true;
                        System.out.println("-1");
                        break;
                    }
                } else {
                    flag = true;
                    System.out.println("-1");
                    break;
                }
                if (!flag) {
                    System.out.println(num);
                }
            }

        } else {
            System.out.println("-1");
        }
    }
}