/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        // your code goes here
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(br.readLine());
        if(n==1){
            System.out.println(-1);
            return;
        }
        String[] lines = br.readLine().split(" ");
        int ans = Integer.parseInt(lines[1]) - Integer.parseInt(lines[0]);
        for(int i=1;i<n;i++){
            lines = br.readLine().split(" ");
            if(Integer.parseInt(lines[1]) - Integer.parseInt(lines[0]) != ans){
                System.out.println(-1);
                return;
            }
        }
        System.out.println(ans>0?ans:-1);
    }
}