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 = false; var odd = false;
        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 = true;
            else odd = true;
            amax = Math.Max(amax, w);
        }
        if ((amax % 2 == 0 && odd) | (amax % 2 == 1 && even)) { Console.WriteLine(-1); goto end; }
        var ans = (asum - amax) / 2L;
        Console.WriteLine(ans);
        end:;
    }
}