#include <iostream>
#include <algorithm>
#include <vector>
#include<string>

using namespace std;
int main(){
	int n;
	cin >> n;
	vector<int> a(n);
	vector<int> b(n);
	for (int i = 0; i < n; i++) cin >> a[i] >> b[i];

	int ans,check;
	bool flag = true;
	for (int i = 0; i < n; i++) {
		ans = b[i] - a[i];
		if (i != 0 && ans != check) {
			flag = false;
			break;
		}
		if (i == 0 && ans <= 0) {
			flag = false;
			break;
		}
		check = ans;
	}
	if (n == 1)flag = false;
	if (flag) cout << ans << endl;
	else cout << -1 << endl;
	return 0;
}