#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
  #define _GLIBCXX_DEBUG
#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;
  set<int> s;
  rep(i, n) {
    int x, y;
    cin >> x >> y;
    s.insert(y - x);
  }
  cout << (s.size() == 1 && *s.begin() > 0 ? *s.begin() : -1) << endl;
  return 0;
}