import java.util.HashMap;
import java.util.Map;
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());

        Map<Long, Long> map = new HashMap<>();
        for (int i = 0; i < N; i++) {
            long X = Long.parseLong(sc.next());
            long Y = Long.parseLong(sc.next());

            if (Y - X > 0) {
                if (!map.containsKey(Y-X)) map.put(Y-X, (long) 0);
                map.put(Y-X, map.get(Y-X)+1);
            }
        }
        if (map.size() == 1) {
            map.forEach((aLong, aLong2) -> System.out.println(aLong));
        } else {
            System.out.println("-1");
        }
    }
}