#include <iostream>
using namespace std;
int main() {
	int n;cin>>n;
	int a,b,c=-1;
	for(int i=0;i<n;i++)
	{
		cin>>a>>b;
		if(c==-1)c=b-a;
		else if(c!=b-a){cout<<-1<<endl;return 0;}
		if(c<=0){cout<<-1<<endl;return 0;}
	}
	cout<<c<<endl;
}