#include <bits/stdc++.h>
#define int long long
#define double long double
using namespace std;
const int MOD = 1000000007;
const int INF = 1e14;
using Graph = vector<vector<int>>;

signed main(){
  int N;
  cin >> N;
  int A, B;
  cin >> A >> B;
  int ans = B-A;
  if( ans <= 0 ){
    cout << -1 << endl;
    return 0;
  }
  for( int i = 1; i < N; i++ ){
    cin >> A >> B;
    if( B-A != ans ){
      cout << -1 << endl;
      return 0;
    }
  }
  cout << ans << endl;
}