#include <iostream>
#include <sstream>
#include <math.h>
#include <stack>
#include <set>
#include <algorithm>
#include <string>
#include <queue>
#include <stack>
#include <map>
#include <cstdio>
#include <vector>
using namespace std;

#define INF (1 << 30)
#define INFLL (1LL << 60)

int main() {
	int n;
	int a,b;
	int num[1001];
	bool ki = 0,gu = 0;
	cin >> n;
	int maxi = 0;
	for(int i = 0;i < n;i++){
		cin >> a >> b;
		num[i] = a + 4*b;
		maxi = max(maxi,num[i]);
		if(num[i] % 2 == 0) gu = 1;
		else ki = 1;
	}
	long long ans = 0;
	sort(num,num+n);
	if(gu && ki) {
		cout << "-1" << endl;
	}else{
		for(int i = 0;i < n;i++){
			ans += maxi - num[i];
		}
		cout << ans/2 << endl;
	}
	return 0;
}