#include<bits/stdc++.h>
using namespace std;
using Int = long long;
//INSERT ABOVE HERE
signed main(){
  Int n;
  cin>>n;
  Int ans=-1;
  for(Int i=0;i<n;i++){
    Int x,y;
    cin>>x>>y;
    if(x>=y){
      cout<<-1<<endl;
      return 0;
    }
    if(!i) ans=y-x;
    if(ans!=y-x){
      cout<<-1<<endl;
      return 0;
    }
  }
  cout<<ans<<endl;
  return 0;
}