#include <bits/stdc++.h>
using namespace std;

#ifdef DEBUG_
#include <compe/debug.hpp>
#else
#define dump(...)
#endif

#define FastIO cin.tie(nullptr), ios_base::sync_with_stdio(false);
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define out(msg) cout << (msg) << '\n'
#define die(msg)         \
  do {                   \
    cout << msg << endl; \
    exit(0);             \
  } while (0)
#define all(k) k.begin(), k.end()
#define rall(k) k.rbegin(), k.rend()
#define INFi 1 << 30
#define INFll 1LL << 60

template <typename T> bool chmax(T& a, const T& b) {
  return ((a < b) ? (a = b, true) : (false));
}
template <typename T> bool chmin(T& a, const T& b) {
  return ((a > b) ? (a = b, true) : false);
}

using llint = long long int;

int main() {
  FastIO;
  int n;
  cin >> n;

  llint max_space{};
  vector<llint> spaces;
  rep(_, n) {
    llint a, b;
    cin >> a >> b;
    llint space = a + 4 * b;
    spaces.push_back(space);
    chmax(max_space, space);
  }

  dump(max_space);
  dump(spaces);

  llint ans{};
  rep(i, n) {
    llint diff = max_space - spaces[i];
    if (diff % 2 != 0) die(-1);
    ans += diff / 2;
  }
  out(ans);
}