#include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<stdio.h>
#include<cmath>
#include<vector>
#include<utility>
#include<stack>
#include<queue>
#include<list>
#include<bitset>

#define FOR(i, a, b) for(int i=(a);i<=(b);i++)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define MOD 1000000007
#define INF 1000000000

using namespace std;


int main(void) {
	int n;
	int point;
	bool flag = false;

	cin >> n;
	FOR(i, 1, n) {
		int a, b;
		cin >> a >> b;
		if (i == 1) {
			point = b - a;
		}
		else {
			if (b - a != point&&flag==false) {
				point = -1;
				flag = true;
			}
		}
	}
	cout << point << endl;
	return 0;
}