import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int ans = -(sc.nextInt() - sc.nextInt());
        if (ans <= 0) {
            System.out.println(-1);
            return;
        }
        for (int i = 1; i < n; i++) {
            if (ans != -(sc.nextInt() - sc.nextInt())) {
                System.out.println(-1);
                return;
            }
        }
        System.out.println(ans);
    }
}