#include <bits/stdc++.h>
using namespace std;
int main(){
  int N;
  cin >> N;
  vector<int> X(N), Y(N);
  for (int i = 0; i < N; i++){
    cin >> X[i] >> Y[i];
  }
  int ans = Y[0] - X[0];
  bool ok = true;
  for (int i = 1; i < N; i++){
    if (ans != Y[i] - X[i]){
      ok = false;
    }
  }
  if (ans <= 0){
    ok = false;
  }
  if (!ok){
    cout << -1 << endl;
  } else {
    cout << ans << endl;
  }
}