#include<bits/stdc++.h>
using namespace std;

int main(){
	cin.tie(0),ios::sync_with_stdio(false);
	int n;
	cin>>n;
	int a,b,ans;
	cin>>a>>b;
	ans=b-a;
	if(ans<=0)ans=-1;
	for(int i=1,x,y;i<n;++i){
		cin>>x>>y;
		if(ans!=y-x)ans=-1;
	}
	cout<<ans<<"\n"s;
}