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

#define rep(i,n) for(int i=0;i<(n);++i)
#define loop for(;;)
#define trace(var) cerr<<">>> "<<#var<<" = "<<var<<endl;
#define inf (1e9)
#define eps (1e-9)
using Integer = long long;
using Real = long double;
const Real PI = acosl(-1);

template<class S, class T> inline
ostream& operator<<(ostream&os, pair<S,T> p) {
  return os << '(' << p.first << ", " << p.second << ')';
}

template<class S, class T, class U> inline
ostream& operator<<(ostream&os, tuple<S,T,U> t) {
  return os << '('
    << get<0>(t) << ", "
    << get<1>(t) << ", "
    << get<2>(t) << ')';
}

template<class T> inline
ostream& operator<<(ostream&os, vector<T> v) {
  if (v.size() == 0) { return os << "(empty)"; }
  os << v[0];
  for (int i=1, len=v.size(); i<len; ++i) os << ' ' << v[i];
  return os;
}

template<class T> inline
istream& operator>>(istream&is, vector<T>&v) {
  rep (i, v.size()) is >> v[i];
  return is;
}

//           ^   >  v   <
int dx[] = { -1, 0, 1,  0 };
int dy[] = {  0, 1, 0, -1 };

using vi = vector<int>;
using vvi = vector<vi>;
using vd = vector<double>;
using vvd = vector<vd>;
using vb = vector<bool>;

int pt[200];
int sc[200];

int main() {
  cin.tie(0);
  ios::sync_with_stdio(0);
  cout.setf(ios::fixed);
  cout.precision(10);
  random_device dev;
  mt19937 rand(dev());

  int n; cin >> n;
  int x, y; cin >> x >> y;
  int ans = y - x;
  rep (i, n-1) {
    int x, y; cin >> x >> y;
    if (ans != y-x) {
      cout << -1 << endl;
      return 0;
    }
  }
  if (ans < 0) {
    cout << -1 << endl;
  } else {
    cout << ans << endl;
  }

  return 0;
}