import java.util.Scanner;

public class Test {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt(), a = sc.nextInt(), b = sc.nextInt(), d = b - a;
		if(d <= 0){
			System.out.println(-1);
			return;
		}
		for(int i = 1; i < n; i++){
			a = sc.nextInt();
			b = sc.nextInt();
			if(d != b-a){
				System.out.println(-1);
				return;
			}
		}
		System.out.println(d);
	}
}