#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream> 
#include<map>
#include <iomanip>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
using namespace std;
#define MAX_MOD 1000000007
#define REP(i,n) for(long long i = 0;i < n;++i)
#define LONG_INF 100000000000000
int main() {
	int n;
	cin >> n;
	vector<long long> s;
	REP(i, n) {
		long long a, b;
		cin >> a >> b;
		a += b * 4;
		s.push_back(a);
	}
	sort(s.begin(), s.end(), greater<long long>());
	long long ans = 0;
	for (int i = 1;i < n;++i) {
		if ((s[0]-s[i]) % 2 == 1) {
			cout << -1 << endl;
			return 0;
		}
		ans += (s[0]-s[i]) / 2;
	}
	cout << ans << endl;
	return 0;
}