using System;

public class Hello
{
    public static void Main()
    {
        var n = int.Parse(Console.ReadLine().Trim());
        var a = new int[n];
        var amax = 0;
        for (int i = 0; i < n; i++)
        {
            string[] line = Console.ReadLine().Trim().Split(' ');
            a[i] = int.Parse(line[0]) + 4 * int.Parse(line[1]);
            amax = Math.Max(amax, a[i]);
        }
        var ans = 0;
        for (int i = 0; i < n; i++)
        {
            var w = amax - a[i];
            if (w % 2 == 1) { Console.WriteLine(-1); goto end; }
            else ans += w / 2;
        }
        Console.WriteLine(ans);
        end:;
    }
}