import java.util.Arrays;
import java.util.Scanner;
public class Main{
	private static Scanner sc=new Scanner(System.in);
	public static void main(String args[])throws Exception{
		int n=sc.nextInt(),are[]=new int[n];
		for(int i=0;i<n;i++){
			are[i]=-(sc.nextInt()-sc.nextInt());
		}Arrays.sort(are);
		int min=are[0];
		if(min<=0)System.out.println(-1);
		else{
			for(int i=0;i<are.length;i++){
				if(min != are[i]){
					System.out.println(-1);
					return;
				}
			}System.out.println(min);
		}
	}
}