#include <iostream>
using namespace std;

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