import java.util.*;

class Main {
	static Scanner sc = new Scanner(System.in);
	public static void main(String[] args) {
		
		int n = sc.nextInt();
		HashSet<Integer> set = new HashSet<>();
		for (int i=0; i<n; i++) {
			int a = sc.nextInt(); int b = sc.nextInt();
			if (b-a < 1) {
				System.out.println(-1); return;
			}
			set.add(b-a);
		}
		
		System.out.println(set.size()>=2?-1:set.stream().findFirst().get());
		
	}
}