// No.178 美しいWhitespace (1)
// https://yukicoder.me/problems/no/178
//
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
using namespace std;

long solve(vector<int> &codes);


int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

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

    long ans = solve(codes);
    cout << ans << endl;
}

long solve(vector<int> &codes) {
    sort(codes.rbegin(), codes.rend());

    long ans = 0;
    int ref = codes[0];
    for (auto c: codes) {
        int diff = ref - c;
        if (diff % 2 != 0) {
            ans = -1;
            break;
        }
        ans += diff / 2;
    }
    return ans;
}