#include <algorithm>
#include <array>
#include <cmath>
#include <cstdio>
#include <iostream>
#include <numeric>
#include <optional>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;

int main() {
    int32_t n;
    cin >> n;
    vector<int64_t> sums(n);
    int64_t r = 0;
    for (auto i = 0; i < n; ++i) {
        int64_t a, b;
        cin >> a >> b;
        auto s = a + 4 * b;
        sums[i] = s;
        r += s % 2;
    }
    if (!(r == 0 || r == n)) {
        cout << -1 << endl;
        return 0;
    }
    int64_t mx = *max_element(sums.begin(), sums.end());
    int64_t ans = 0;
    for (auto s : sums) {
        ans += mx - s;
    }
    cout << ans / 2 << endl;
    return 0;
}