#include <bits/stdc++.h>

using namespace std;

int main()
{
  int N, X[1000], Y[1000];

  cin >> N;
  for(int i = 0; i < N; i++) {
    cin >> X[i] >> Y[i];
  }
  int ret = max(-1, Y[0] - X[0]);
  for(int i = 0; i < N; i++) {
    if(Y[i] - X[i] != ret) ret = -1;
  }
  if(ret == 0) ret = -1;
  cout << ret << endl;
}