#include <iostream>

using namespace std;

int main() {
  int n;
  cin >> n;
  int x, y, tmp;
  cin >> x >> y;
  tmp=y-x;
  for (int i=1; i<n; i++) {
    cin >> x >> y;
    if (tmp<=0 || (tmp!=y-x)) {
      tmp=-1;
    } else {
      tmp=y-x;
    }
  }
  
  cout << (tmp>0 ? tmp: -1) << endl;
  return 0;
}