import java.util.ArrayList;
import java.util.Scanner;

public class Wahu {

    public static void main(String[] args) {
	Scanner s = new Scanner(System.in);
	int N = Integer.parseInt(s.nextLine());
	ArrayList<Integer> list = new ArrayList<>();
	for(int i=0;i<N;i++){
	    int a = s.nextInt();
	    int b = s.nextInt();
	    list.add(b-a);
	}
	int buff = list.get(0);
	if(buff <= 0){
	    System.out.println(-1);
	    return;
	}
	for(int i=1;i<list.size();i++){
	    if(buff != list.get(i)){
		System.out.println(-1);
		return;
	    }
	}
	System.out.println(buff);
	return;
    }

}