#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <stack>
#include <algorithm>
#include <string>
#include <map>
#include <iterator>
#include <set>
#include <queue>
#include <bitset>
#include <cassert>

using namespace std;

int main() {

    int N;
    cin >> N;
    int a, b;
    cin >> a >> b;
    vector<int> vs;
    int m = a + 4 * b;
    vs.push_back(a + 4 * b);
    for (int i = 1; i < N; ++i) {
        cin >> a >> b;
        m = max(m, a + 4 * b);
        vs.push_back(a + 4 * b);
    }

    for (int i = 0; i < N; ++i) {
        if ((m % 2) != (vs[i] % 2)) {
            cout << -1 << endl;
            return 0;
        }
    }
    long ans = 0;
    for (int i = 0; i < N; ++i) {
        ans += (m - vs[i]) / 2;
    }
    cout << ans << endl;


    return 0;

}