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()); int ans = 0; int tmp = 0; int X; int Y; for (int i = 0; i < N; i++) { X = Integer.parseInt(sc.next()); Y = Integer.parseInt(sc.next()); ans = Y - X; if (ans <= 0 || ans != tmp) { ans = -1; break; } tmp = ans; } System.out.println(ans); } }