#define _USE_MATH_DEFINES
#include<stdio.h>
#include<string>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include<iomanip>
//#include<bits/stdc++.h>


using namespace std;




int main() {
  
	int n;
	cin>>n;
	vector<int>p(n);
	for(int i=0;i<n;i++){
		int a,b;
		cin>>a>>b;
		p[i]=a+4*b;
	}
	sort(p.begin(),p.end());
	long long int t=0;
	for(int i=0;i<n-1;i++){
		if((p[n-1]%2&&p[i]%2)||(p[n-1]%2==0&&p[i]%2==0)){
			t+=(p[n-1]-p[i])/2;
		}
		else{
			cout<<"-1"<<endl;
			return 0;
		}
	}
	cout<<t<<endl;




    return 0;
}