import java.util.Scanner;

public class No185 {

    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        int count = s.nextInt();
        int diff = -1;
        for (int i = 0; i < count; i++) {
            if (i == 0) {
                diff = -s.nextInt() + s.nextInt();
                if (diff < 1) {
                    diff = -1;
                }
            } else if (-s.nextInt() + s.nextInt() != diff) {
                diff = -1;
                break;
            }
        }
        System.out.println(diff);
    }
}