class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            decimal[] size = new decimal[n];
            for (int i = 0; i < n; i++)
            {
                decimal[] code = Array.ConvertAll(Console.ReadLine().Split(' '), num => decimal.Parse(num));
                size[i] = code[0] + (code[1]*4);
            }
            Array.Sort(size);
            Array.Reverse(size);
            decimal minsize = 0;
            if((size[0] - size[1]) %2 == 0)
            {
                minsize = ShortageCheck(size);
                if (minsize != -1)
                {
                    minsize = minsize / 2;
                }
            }
            else
            {
                minsize = -1;
            }
            Console.WriteLine(minsize);
        }

        private static decimal ShortageCheck(decimal[] size)
        {
            decimal shortage = 0;
            for(int i = 1; i < size.Length; i++)
            {
                if ((size[0] - size[i])%2 == 0)
                {
                    shortage += size[0] - size[i];
                }
            }
            return shortage;
        }
    }