using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Programming
{
    class Program
    {
        static void Main(string[] args)
        {
            //入力
            int N = int.Parse(Console.ReadLine());

            int[,] a = new int[N, 2];
            string[] str = new string[2];

            for (int i = 0; i < N; i++)
            {
                str = Console.ReadLine().Split(' ');
                a[i, 0] = int.Parse(str[0]);
                a[i, 1] = int.Parse(str[1]);
            }

            //処理
            int maxLength = 0;

            //最長文字数を計算
            for (int i = 0; i < N; i++)
            {
                if (maxLength < a[i, 0] * 1 + a[i, 1] * 4) maxLength = a[i, 0] * 1 + a[i, 1] * 4;
            }

            int length;
            int count = 0;
            bool isError = false;

            for (int i = 0; i < N; i++)
            {
                length = a[i, 0] * 1 + a[i, 1] * 4;
                while (length < maxLength)
                {
                    length = length + 2;
                    count++;
                }

                if (length > maxLength) isError = true;
            }

            //出力
            if (isError)
            {
                Console.WriteLine(-1);
            }
            else
            {
                Console.WriteLine(count);
            }
        }
    }
}