using System;
using System.Collections.Generic;
using System.Linq;

public class yukicoder
{
    
    public static decimal moji(string s)
    {
        decimal[] length = s.Split(' ').Select(x => decimal.Parse(x)).ToArray();
        
        return length[0] + (length[1] * 4);
    }
    
    public static void Main()
    {
        decimal size = decimal.Parse(Console.ReadLine());
        List<decimal> Length = new List<decimal>();
        
        for(int i = 0; i <= size - 1; i++)
        {
            Length.Add(moji(Console.ReadLine()));
        }
        
        decimal count = 0;
        decimal max = Length.Max();
        
        if(Length.Any(x => (max - x) % 2 != 0))
        {
            Console.WriteLine("-1");
            return;
        }

        for(int i = 0; i <= Length.Count - 1; i++)
        {
            count += (max - Length[i]) / 2;
        }
        
        Console.WriteLine(count);
        
    }
}