#include <bits/stdc++.h>
#define rep(i, a, n) for (int i = a; i < n; i++)
#define repr(i, a, n) for (int i = n - 1; i >= a; i--)
using namespace std;
using ll = long long;
using P = pair<int, int>;
template <typename T> void chmin(T &a, T b) { a = min(a, b); }
template <typename T> void chmax(T &a, T b) { a = max(a, b); }

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  int n, bef = 0;
  cin >> n;
  bool is_ok = true;
  rep(i, 0, n) {
    int x, y;
    cin >> x >> y;
    int z = y - x;
    if (z <= 0) {
      is_ok = false;
      break;
    }
    if (i == 0)
      bef = z;
    else {
      if (bef != z) {
        is_ok = false;
        break;
      }
    }
  }
  if (is_ok)
    cout << bef << endl;
  else
    cout << -1 << endl;
}