using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace yukicoder_185 {
    class Program {
        static void Main(string[] args) {
            int n = int.Parse(Console.ReadLine());
            int[,]s = new int[n, 2];
            int check = 0;
            for (int i = 0; i < n; i++) {
                string[] t = Console.ReadLine().Split(' ');
                s[i, 0] = int.Parse(t[0]);
                s[i, 1] = int.Parse(t[1]);
                if (s[i, 1] - s[i, 0] < 0) {
                    check++;
                }
            }
            if (check != 0) {
                Console.WriteLine(-1);
            } else {
                if (n == 1) {
                        Console.WriteLine(s[0, 1] - s[0, 0]);
                }else {
                    for (int i = 1; i < n; i++) {
                        if (s[i, 1] - s[i, 0] != s[i - 1, 1] - s[i - 1, 0]) {
                            check++;
                            break;
                        }
                    }
                    if (check != 0) {
                        Console.WriteLine(-1);
                    } else {
                        Console.WriteLine(s[0, 1] - s[0, 0]);
                    }
                }
            }
            Console.ReadLine();
        }
    }
}