#include <bits/stdc++.h>
#define rep(i, a) for (int i = 0; i < (a); i++)
#define rep2(i, a, b) for (int i = (a); i < (b); i++)
#define repr(i, a) for (int i = (a) - 1; i >= 0; i--)
#define repr2(i, a, b) for (int i = (b) - 1; i >= (a); i--)
using namespace std;
typedef long long ll;
const ll inf = 1e9;
const ll mod = 1e9 + 7;

ll a[1000];
ll b[1000];

int main() {
    int N;
    cin >> N;
    rep (i, N) cin >> a[i] >> b[i];
    rep (i, N) a[i] += 4 * b[i];

    bool even = false, odd = false;
    rep (i, N) {
        if (a[i] % 2 == 0) even = true;
        else odd = true;
    }

    if (even && odd) {
        cout << -1 << endl;
        return 0;
    }

    ll maxv = *max_element(a, a + N);

    ll ans = 0;
    rep (i, N) {
        ans += (maxv - a[i]) / 2;
    }

    cout << ans << endl;
}