#include<iostream>
#include<cstdio>
#include<vector>
#include<queue>

#include<map>
#include<set>
#include<string>
#include<algorithm>
#include<functional>
using namespace std;
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#define INF 1<<30
#define MP make_pair
#define mp make_pair
#define pb push_back
#define PB push_back
#define DEBUG(x) cout<<#x<<": "<<x<<endl
#define ll long long
#define ull unsigned long long

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	int n;cin>>n;
	set<int> a;
	REP(i,n){
		int x,y;cin>>x>>y;
		a.insert(y-x);
	}
	if(a.size()==1 && *a.begin()>0){
		cout<<*a.begin()<<endl;
	}else{
		cout<<-1<<endl;
	}

}