#include <bits/stdc++.h>

using namespace std;

#define LL long long
#define REP(i, n) for(int i = 0; i < n; i++)
#define ALL(a) (a).begin(),(a).end()

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    int n;
    cin >> n;
    vector<LL> ws(n);
    REP(i, n) {
        LL a, b;
        cin >> a >> b;
        ws[i] = a + 4 * b;
    }

    LL wsMax = *std::max_element(ALL(ws));
    LL ans = 0;
    REP(i, n) {
        if ((wsMax - ws[i]) % 2 != 0) {
            cout << -1 << endl;
            return 0;
        } else {
            ans += (wsMax - ws[i]) / 2;
        }
    }
    cout << ans << endl;
    return 0;
}