#include <bits/stdc++.h>

#define forn(i, n) for (int i = 0; i < (int)(n); ++i)
#define for1(i, n) for (int i = 1; i <= (int)(n); ++i)
#define fore(i, l, r) for (int i = (int)(l); i <= (int)(r); ++i)
#define ford(i, n) for (int i = (int)(n) - 1; i >= 0; --i)
#define pb push_back
#define fi first
#define se second
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vi> vvi;
typedef vector<vll> vvl;
typedef pair<int, int> pint;
typedef vector<pint> vpint;
typedef long double ld;

int main() {
  int N;
  cin >> N;

  vll A(N);
  forn (i, N) {
    ll a, b, s;
    cin >> a >> b;
    s = a + 4 * b;
    A[i] = s;
  }

  ll mx = *max_element(all(A)), ans = 0;
  forn (i, N) {
    ll dis = mx - A[i];
    if (dis % 2) {
      cout << -1 << endl;
      return 0;
    }
    ans += (dis / 2);
  }
  cout << ans << endl;
  return 0;
}