//#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
//using namespace atcoder;
using ll = long long;
#define all(A) A.begin(),A.end()
using vll = vector<ll>;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
using Graph = vector<vector<ll>>;
Graph G;
vll dist;
vector<bool> seen;
ll gcd(ll(a), ll(b)) {
	ll c = a;
	while (a % b != 0) {
		c = a % b;
		a = b;
		b = c;
	}
	return b;
}
int main() {
	ll N;
	cin>>N;
	ll X,Y;
	cin>>X>>Y;
	ll an=Y-X;
	if(an<=0){
		cout<<-1<<endl;
			return 0;
	}
	rep(i,N-1){
		cin>>X>>Y;
		if(Y-X!=an){
			cout<<-1<<endl;
			return 0;
		}
	}
	cout<<an<<endl;
}