#include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <cstdio>
#include <functional>
#include <set>
#include <sstream>
#include <map>
#include <queue>
#include <stack>

using namespace std;


int main()
{
	int n;
	cin>>n;

	vector<long long> len(n);
	for(int i=0;i<n;i++){
		long long a,b;
		cin>>a>>b;

		len[i]=a+4*b;
	}

	sort(len.begin(),len.end());

	long long res=0;
	for(int i=0;i<n-1;i++){
		if((len[n-1]-len[i])%2){cout<<-1<<endl; return 0;}
		res+=(len[n-1]-len[i])/2;
	}

	cout<<res<<endl;

	return 0;
}