#include <iostream>
using namespace std;

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