#include<bits/stdc++.h>

#define X first
#define Y second
#define eb push_back
#define siz(a) int(a.size())
#define endl "\n"

#define trace2(x, y)             cerr <<#x<<": "<<x<<" | "<<#y<<": "<<y<< endl;
#define trace3(x, y, z)          cerr <<#x<<": "<<x<<" | "<<#y<<": "<<y<<" | "<<#z<<": "<<z<<endl;
#define trace4(a, b, c, d)       cerr <<#a<<": "<<a<<" | "<<#b<<": "<<b<<" | "<<#c<<": "<<c<<" | "<<#d<<": "<<d<<endl;
#define trace5(a, b, c, d, e)    cerr <<#a<<": "<<a<<" | "<<#b<<": "<<b<<" | "<<#c<<": "<<c<<" | "<<#d<<": "<<d<<" | "<<#e<<": "<<e<<endl;

using namespace std;

typedef long long int ll;
typedef vector < int > vi;
typedef vector < ll > vll;

const int mod=1e9+7;
const int maxn=1e6+5;

/*	finding inverse upto n
 * for i in xrange(2, N):
    ifc[i] = -(mod / i) * ifc[mod % i] % mod
 * 
*/
ll power(ll base, ll exp, ll mod){ll res = 1; while(exp){if(exp%2)res*=base;base*=base;res%=mod;base%=mod;exp/=2;}return res;}

ll t, n, v[maxn], x[maxn];
double dp[2][55];

vector<pair<ll, ll> > vv;

double go(int idx, bool dir){
	if(idx == n+1)
		return (double)1e18;
	
	double &ret = dp[dir][idx];
	
	if(ret != -1)
		return ret;
	
	for(int i = 0; i<2; i++){
		if(idx == 1){
			ret = max(ret, go(idx+1, i));
		}
	}	
		//left
		if(dir){
			
			double ans = 1e18;
			
			if(v[idx] > v[idx-1]){
				ans = 1.0*(x[idx] - x[idx-1])/(v[idx] - v[idx-1]);
			} else if(v[idx] == v[idx-1]){
				if(x[idx] == x[idx-1])
					ans = 0;
			}
			ret = max(ret, min(go(idx+1, dir), ans));
			
			ret = max(ret, go(idx+1, !dir));
		} else{
			double ans = 1e18;
			
			if(v[idx] < v[idx-1]){
				ans = -1.0*(x[idx] - x[idx-1])/(v[idx] - v[idx-1]);
			} else if(v[idx] == v[idx-1]){
				if(x[idx] == x[idx-1])
					ans = 0;
			}
			
			ret = max(ret, min(go(idx+1, dir), ans));
			
			ans = 1.0*(x[idx] - x[idx-1])/(v[idx] + v[idx-1]);
			
			ret = max(ret , min(go(idx+1, !dir), ans));
		}
		
	return ret;
}

int main(){
	ios_base::sync_with_stdio(false);cin.tie(NULL); cout.tie(NULL);
	//freopen("input.in", "r", stdin);freopen("output.out", "w", stdout);	
	cin >> t;
	
	while(t--){
		cin >> n;
		vv.clear();
		for(int i=1; i<=n; i++){
			cin >> x[i] >> v[i];
			dp[0][i] = -1;
			dp[1][i] = -1;
			vv.eb({x[i], v[i]});
		}
		
		sort(vv.begin(), vv.end());
		
		for(int i=1; i<=n; i++){
			x[i] = vv[i-1].X;
			v[i] = vv[i-1].Y;
		}
		
		double ans = go(1, 0);
		
		if(ans >= 1e17)
			cout << -1 << endl;
		else
			cout << fixed << setprecision(10) <<  ans << endl;
	}
	
	return 0;
}