using System;

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