#include <algorithm>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <ctime>
#include <iostream>
#include <sstream>
#include <functional>
#include <map>
#include <string>
#include <cstring>
#include <vector>
#include <queue>
#include <stack>
#include <deque>
#include <set>
#include <list>
#include <numeric>
using namespace std;
const double PI = 3.14159265358979323846;
const double EPS = 1e-12;
const int INF = 1<<25;
typedef pair<int,int> P;
typedef long long ll;
typedef unsigned long long ull;


int main(){
	int n;
	cin>>n;
	int res = -2;
	for(int i = 0; i < n; i++){
		int x, y;
		cin>>x>>y;
		if(res==-2) res = y-x;
		else {
			if(res==y-x && y-x>0) continue;
			else res = -1;
		}
	}
	cout<<res<<endl;
	return 0;
}