#include<bits/stdc++.h>
using namespace std;
using Int=long long;
using Real=long double;
template<class T,class S>inline bool chmin(T&A,S B){if(A>B){A=B;return true;}else{return false;}}
template<class T,class S>inline bool chmax(T&A,S B){if(A<B){A=B;return true;}else{return false;}}
#define REP(i,n)for(int i=0;i<(n);++i)
#define REP1(i,n)for(int i=1;i<=(n);++i)
#define ALL(x)begin(x),end(x)
#define drop(x){cout<<(x)<<'\n';return;}
int N,X,Y;
void sol()
{
	cin>>N;
	cin>>X>>Y;
	int ans=Y-X;
	REP(i,N-1)
	{
		cin>>X>>Y;
		if(ans!=Y-X)drop(-1);
	}
	if(ans<1)drop(-1);
	cout<<ans<<"\n";
}
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
	cout<<fixed<<setprecision(16);
	int TE=1;
	//cin>>TE;
	while(TE--)sol();
}