#include <bits/stdc++.h>
#define INF 990000000
//35
using namespace std;

int main() {
int n;
cin>>n;
int a,b;
cin>>a>>b;
int tem=b-a;
int flag=0;
for(int i=1;i<n;i++){
cin>>a>>b;
if(tem!=(b-a)){
flag=1;
break;
}
}
if(tem<=0)flag=1;
if(flag==1)
cout <<"-1"<<endl;
else
cout <<tem<<endl;


}