#include <iostream>
#include <fstream>
#include <vector>
#include <string>
#include <algorithm>
using namespace std;

int main(){


		int n,x0,y0; cin >> n;
		cin >> x0 >> y0;
		int tmp = y0 - x0;
		for (int i = 0; i < n - 1; i++) {
				int x,y; cin >> x >> y;
				if (tmp != y - x) {
					tmp = -1;
					break;
				}

		}
		if (tmp <= 0) {
			cout << -1 << "\n";
		}else{
			cout << tmp << "\n";
		}

		return 0;
}