#include <stdio.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <functional>
#include <map>
#include <iomanip>
#include <math.h> 
#include <stack>
#include <queue>
#include <bitset>
#include <cstdlib>
#include <tuple>
#include <cctype>
#include <ctype.h>
#include <set>
#include <sstream>

using namespace std;


int main() {
	int i, j, k;
	int n;
	vector<int>saa;

	cin >> n;

	int sa;

	for (i = 0; i < n; i++) {
		int x, y;
		cin >> x >> y;
		 sa = y - x;
		 if (sa <= 0) {
			 cout << -1 << endl;
			 return 0;
		 }
		saa.push_back(sa);
	}

	for (i = 0; i < saa.size(); i++) {
		if (sa != saa[i]) {
			cout << -1 << endl;
			return 0;
		}
	}

	cout << sa << endl;



	getchar();
	getchar();
	return 0;
}