#include<bits/stdc++.h>

using namespace std;
using ll=long long;
#define rep2(i, a, n) for(int i = (a); i < (n); i++)
#define rep(i, n) rep2(i,0,n)

int main(){
  cin.tie(nullptr);ios_base::sync_with_stdio(false);
  int x,y,n,ans;cin>>n;
  cin>>x>>y;
  if(y-x>0){
    ans=y-x;
    //cout<<ans<<endl;
  }else{
    ans=-1;
  }
  n--;
  for(;n>0;n--){
    cin>>x>>y;
    if(y-x!=ans) ans=-1;
  }
  cout<<ans<<endl;
}