class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            decimal[] size = new decimal[n];
            decimal[] code = Array.ConvertAll(Console.ReadLine().Split(' '), num => decimal.Parse(num));
            size[0] = code[0] + (code[1] * 4);
            decimal checkSize = size[0]%2; //0:奇数　1:偶数　-1:どちらでもない

            for (int i = 1; i < n; i++)
            {
                code = Array.ConvertAll(Console.ReadLine().Split(' '), num => decimal.Parse(num));
                if (checkSize != -1 && (code[0] + (code[1] * 4)) % 2 == checkSize)
                {
                    size[i] = code[0] + (code[1] * 4);
                }
                else
                {
                    checkSize = -1;
                }
            }
            if (checkSize == -1)
            {
                Console.WriteLine(-1);
            }
            else
            {
                Array.Sort(size);
                Array.Reverse(size);
                decimal minsize = ShortageCheck(size);
                //if (minsize != -1)
                //{
                //    minsize = minsize / 2;
                //}
                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];
                }
                //else
                //{
                //    shortage = -1;
                //    break;
                //}
            }
            return shortage;
        }
    }