#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001


int main(){	
	
	int n;
	cin>>n;
	
	vector<int> x(n),y(n);
	rep(i,n)cin>>x[i]>>y[i];
	
	int ans = y[0]-x[0];
	rep(i,n){
		if(x[i]+ans!=y[i]){
			cout<<-1<<endl;
			return 0;
		}
	}
	if(ans<=0)cout<<-1<<endl;
	else cout<<ans<<endl;
	
	return 0;
	
}