using System;
using System.Linq;

class No178{
    static void Main(){
        long max=0;
        long count=0;
        var isEvenNum=false;
        var num=Int32.Parse(Console.ReadLine());
        for(int i=0;i<num;++i){
            var line=Console.ReadLine().Split(' ').Select(x=>Int64.Parse(x)).ToArray();
            var l=line[0]+line[1]*4;
            if(i!=0&&((l%2==0)!=isEvenNum)){
                count=-1;
                break;
            }
            else isEvenNum=l%2==0;
            if(l>max){
                count+=(l-max)*i;
                max=l;
            }
            count+=max-l;
        }
        Console.WriteLine(count>0?count/2:count);
    }
}