#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(ll i = 0, i##_len = (n); i < i##_len; i++)
#define reps(i, s, n) for(ll i = (s), i##_len = (n); i < i##_len; i++)
#define rrep(i, n) for(ll i = (n) - 1; i >= 0; i--)
#define rreps(i, e, n) for(ll i = (n) - 1; i >= (e); i--)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((ll)(x).size())
#define len(x) ((ll)(x).length())

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    // ifstream in("input.txt");
    // cin.rdbuf(in.rdbuf());
    ll n;
    cin >> n;
    ll mv = LONG_LONG_MIN;
    vector<pair<ll, ll>> ab(n);
    rep(i, n) {
        cin >> ab[i].first >> ab[i].second;
        mv = max(mv, ab[i].first + ab[i].second * 4);
    }
    bool f = true;
    ll ans = 0;
    rep(i, n) {
        ll rem = mv - (ab[i].first + ab[i].second * 4);
        if (rem % 2 != 0) {
            f = false;
            break;
        }
        ans += rem / 2;
    }
    if (f) cout << ans << endl;
    else cout << -1 << endl;
    return 0;
}