using System;
using System.Collections.Generic;
using System.Linq;
class Program {
    static void Main() {
        long ans = 0;
        int n = int.Parse(Console.ReadLine());
        List<int> lens = new List<int>();
        for (int i = 0; i < n; i++) {
            int[] ab = Console.ReadLine().Split().Select(int.Parse).ToArray();
            lens.Add(ab[0] + ab[1] * 4);
        }

        int max = lens.Max();

        foreach (int len in lens) {
            int dif = max - len;
            if (dif % 2 == 1) {
                ans = -1;
                break;
            }
            ans += dif / 2;
        }

        Console.WriteLine(ans);
    }
}