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

int main(){
  int N,a,b;
  cin>>N;
  int res[N];
  for(int i=0;i<N;i++){
    cin>>a>>b;
    res[i]=b-a;
    if(i != 0 && res[i] !=res[i-1]){
      cout<<-1<<endl;
      return 0;
    }
  }
  if(res[0]<=0){
    cout<<-1<<endl;
    return 0;
  }
  cout<<res[0]<<endl;
  return 0;
}