using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
class TEST{
	static void Main(){
		Sol mySol =new Sol();
		mySol.Solve();
	}
}

class Sol{
	public void Solve(){
		int[] chk=new int[2];
		int[] Tot=new int[N];
		for(int i=0;i<N;i++){
			chk[(A[i]+4*B[i])%2]++;
			Tot[i]=A[i]+4*B[i];
		}
		if(chk[0]>0&&chk[1]>0){
			Console.WriteLine(-1);
			return;
		}
		int max=Tot.Max();
		long ans=0;
		for(int i=0;i<N;i++)ans+=(max-Tot[i])/2;
		Console.WriteLine(ans);
		
		
		
	}
	int N;
	int[] A;
	int[] B;
	public Sol(){
		N=ri();
		A=new int[N];
		B=new int[N];
		for(int i=0;i<N;i++){
			var d=ria();
			A[i]=d[0];B[i]=d[1];
		}
	}




	static String rs(){return Console.ReadLine();}
	static int ri(){return int.Parse(Console.ReadLine());}
	static long rl(){return long.Parse(Console.ReadLine());}
	static double rd(){return double.Parse(Console.ReadLine());}
	static String[] rsa(){return Console.ReadLine().Split(' ');}
	static int[] ria(){return Array.ConvertAll(Console.ReadLine().Split(' '),e=>int.Parse(e));}
	static long[] rla(){return Array.ConvertAll(Console.ReadLine().Split(' '),e=>long.Parse(e));}
	static double[] rda(){return Array.ConvertAll(Console.ReadLine().Split(' '),e=>double.Parse(e));}
}