#include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<stdio.h>
#include<cmath>
#include<vector>
#include<utility>
#include<stack>
#include<queue>
#include<list>
#include<bitset>
#include<functional>

#define FOR(i, a, b) for(int i=(a);i<=(b);i++)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define MOD 1000000007
#define INF 1000000000

using namespace std;
typedef pair<int, int> P;

int main(void) {
	int n;
	bool flag;
	bool check = true;
	long long int count = 0;
	long long int max = 0;
	long long int p[1001];

	cin >> n;
	FOR(i, 1, n) {
		int a, b;
		cin >> a >> b;
		p[i] = a + 4 * b;
		if (max < p[i]) {
			max = p[i];
		}
	}
	if (p[1] % 2 == 0) {
		flag = true;
	}
	else {
		flag = false;
	}
	FOR(i, 1, n) {
		if (flag) {
			if (p[i] % 2 == 0) {
				count += (max - p[i]) / 2;
			}
			else {
				check = false;
				break;
			}
		}
		else {
			if (p[i] % 2 == 1) {
				count += (max - p[i]) / 2;
			}
			else {
				check = false;
				break;
			}
		}
	}
	if (check) {
		cout << count << endl;
	}
	else {
		cout << -1 << endl;
	}
	return 0;
}