#include <iostream>
#include <vector>
#include <cstdio>
#include <sstream>
#include <map>
#include <string>
#include <algorithm>
#include <queue>
#include <cmath>
#include <set>
using namespace std;

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

	vector<int> a(n), b(n);
	vector<int> l(n);
	for(int i=0; i<n; i++){
		cin >> a[i] >> b[i];
		l[i] = a[i] + 4*b[i];
	}

	bool ok = true;
	int max_ = l[0];
	for(int i=1; i<n; i++){
		if(l[i]%2 != l[i-1]%2) ok = false;
		max_ = max(max_, l[i]);
	}

	if(ok == false){
		cout << -1 << endl;
		return 0;
	}

	long long ans = 0;
	for(int i=0; i<n; i++){
		ans += (max_ - l[i])/2;
	}

	cout << ans << endl;

	return 0;
}