import java.util.*;
class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int ans = -1;
		for(int i = 0; i < n; i++) {
			int a = sc.nextInt();
			int b = sc.nextInt();
			
			int c = b - a;
			
			if(i == 0 && c > 0) {
				ans = c;
			}
			else if(ans != c) {
				ans = -1;
			}
			else if(c <= 0) {
				ans = -1;
			}
		}
		System.out.println(ans);
	}
}