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

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