#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef tuple<int, int, int> tii;

#define eb emplace_back
#define all(s) (s).begin(), (s).end()
#define rep(i, a, b) for (int i = (a); i <= (b); ++i)
#define per(i, a, b) for (int i = (a); i >= (b); --i)

inline int rd() {
    int x = 0;
    bool f = 0;
    char c = getchar();
    for (; !isdigit(c); c = getchar()) f |= (c == '-');
    for (; isdigit(c); c = getchar()) x = x * 10 + (c ^ 48);
    return f ? -x : x;
}

const int N = 1000007;

int c[N], t[N];

pll s[N];

inline void work() {
    int n = rd();
    bool fl = false;
    rep(i, 1, n) {
        t[i] = rd(); c[i] = rd();
        if (i > 2 && t[i - 2] == t[i]) fl = true;
    }
    if (fl) {puts("-1"); return;}
    auto check = [&](ll spd) {
        ll L = -1e18, R = 1e18;
        rep(i, 2, n) {
            ll D = spd * (t[i] - t[i - 1]);
            if (abs(c[i] - c[i - 1]) > D) {
                if (c[i] < L - D || c[i] > R + D) return false;
                L = R = c[i - 1];
            }
            L -= D; R += D;
        }
        return true;
    };
    int L =  0, R = 1000000000;
    while (L < R) {
        int m = (L + R) / 2;
        check(m) ? R = m : L = m + 1;
    }
    printf("%d\n", L);
}

int main() {
    per(t, rd(), 1) work();
    return 0;
}