#include<iostream>
#include<cmath>
#include<string>

using namespace std;

int main(){

	int N,X,Y;
	int ans=-1;
	int dame=1;

	cin>>N;

	for(int i=0;i<N;i++){
		cin>>X>>Y;
		if(Y-X>0){
			if(ans==-1) ans=Y-X;
			else if(ans!=Y-X) dame=-1;
		}
		else dame=-1;
	}

	if(dame==-1) cout<<"-1"<<endl;
	else cout<<ans<<endl;
}