#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  int n;
  cin >> n;
  vector<pair<int, int>> v(n);
  rep(i, n) cin >> v[i].first >> v[i].second;
  int maxim = 0;
  for (auto [a, b] : v) maxim = max(maxim, a + 4 * b);
  int ans = 0;
  for (auto [a, b] : v) {
    int x = a + 4 * b;
    if (x % 2 != maxim % 2) {
      cout << -1 << endl;
      return 0;
    }
    ans += (maxim - x) / 2;
  }
  cout << ans << endl;
  return 0;
}