#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(n);++i)
#define all(a) (a).begin(),(a).end()
using namespace std;
typedef long long ll;

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	int n;
	cin >> n;
	vector<ll> a(n);
	vector<ll> b(n);
	ll res = 0;
	rep(i, n) {
		cin >> a[i] >> b[i];
	res = max(res, a[i] + 4 * b[i]);
	}
	//a + 4*b + 2*c
	ll ans = 0;
	rep(i, n) {
		if ((res - a[i] - 4 * b[i]) % 2 == 0) {
			ans += (res - a[i] - 4 * b[i]) / 2;
		}
		else {
			cout << -1 << endl;
			return 0;
		}
	}
	cout << ans << endl;
	return 0;
}