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

class Magatro
{
    static void Main()
    {
        long ans = 0;
        int N = int.Parse(Console.ReadLine());
        long[] a = new long[N];
        for(int i = 0; i < N; i++)
        {
            string[] s = Console.ReadLine().Split(' ');
            int w = int.Parse(s[0]);
            w += int.Parse(s[1]) * 4;
            a[i] = w;
        }
        long max = a.Max();
        for(int i = 0; i < N; i++)
        {
            long p = (max - a[i]);
            if (p % 2 == 1)
            {
                Console.WriteLine(-1);
                return;
            }
            else
            {
                ans += p / 2;
            }
        }
        Console.WriteLine(ans);
    }
}