#include <bits/stdc++.h>
using namespace std;

const int N = 1e6 + 5;

int T, n, m, t[N], c[N];

bool check(int v) {
	int l = 1, r = 1e9, p = c[1];
	for (int i = 2; i <= n; i++) {
		if (t[i] == t[i - 1]) {
			if (r < c[i] || l > c[i]) {
				return 0;
			}
			l = c[i];
			r = c[i];
		} else {
			int d = t[i] - t[i - 1];
			int la = max(l - d * v, 1), ra = min(r + d * v, (int)1e9);
			int lb = max(p - d * v, 1), rb = min(p + d * v, (int)1e9);
			bool ina = c[i] >= la && c[i] <= ra, inb = c[i] >= lb && c[i] <= rb;
			if (!ina && !inb) {
				return 0;
			} else if (!ina) {
				l = la, r = ra;
			} else if (!inb) {
				l = lb, r = rb;
			} else {
				l = min(la, lb), r = max(ra, rb);
			}
			p = c[i];
		}
	}
	return 1;
}

void work() {
	cin >> n;
	for (int i = 1; i <= n; i++) {
		cin >> t[i] >> c[i];
	}
	for (int i = 1; i + 2 <= n; i++) {
		if (t[i] == t[i + 2]) {
			cout << "-1\n";
			return;
		}
	}
	int l = 0, r = 1e9;
	while (l < r) {
		int mid = l + r >> 1;
		if (check(mid)) {
			r = mid;
		} else {
			l = mid + 1;
		}
	}
	cout << l << '\n';
}

int main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	cin >> T;
	while (T--) {
		work();
	}

	return 0;
}