class Program
{
    static void Main(string[] args)
    {
        string inputCount = Console.ReadLine()!;
        int count = int.Parse(inputCount);
        int ansCopy = 0;
        
        for (int i = 0; i < count; i++)
        {
            string[] inputNum = Console.ReadLine()!.Split(' ');
            int leftNum = int.Parse(inputNum[0]);
            int rightNum = int.Parse(inputNum[1]);

            if (rightNum <= leftNum)
            {
                Console.WriteLine(-1);
                break;
            }
            else if (i == 0)
            {
                ansCopy = rightNum - leftNum;
            }
            else
            {
                if (ansCopy != rightNum - leftNum)
                {
                    Console.WriteLine(-1);
                    break;
                }
            }

            if (i == count - 1)
            {
                Console.WriteLine(ansCopy);
            }
        }
    }
}