#define _USE_MATH_DEFINES
#include  <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
#include<math.h>
#include<iomanip>
#include<stdio.h>
#include <stdlib.h>
#include<stdio.h>
#include <queue>
#include<map>
#include <fstream>
#include <sstream>
#include<set>
#include<stack>
#include<time.h>
#include <sys/types.h>
#include <tuple>
#include <numeric>
#include <iterator>

typedef long long int ll;
//#include<bits/stdc++.h>


using namespace std;

int gcd(int a, int b) {
    if (a < b)swap(a, b);
    if (b == 0)return a;
    return gcd(b, a % b);
}
int lcm(int a, int b) {
    return a * b / gcd(a, b);
}


int main()
{

    int Q;
    Q = 1;
    //cin >> Q;


    while (Q--) {
        
        ll n;
        cin >> n;
        vector<ll>p(n);
        for (int i = 0; i < n; i++) {
            int a, b;
            cin >> a >> b;
            p[i] = a + 4 * b;
        }
        sort(p.rbegin(), p.rend());
        ll ans = 0;
        for (int i = 1; i < n; i++) {
            
            if ((p[0] - p[i]) % 2) {
                cout << -1; return 0;
            }
            if (p[0] && !p[i]) {
                cout << -1;
                return 0;
            }
            ans += p[0] - p[i];
        }
        cout << ans / 2;
    }
    return 0;
}