/*
  Beautiful Codes are MUCH better than 'Shorter' ones !
user  : triveni
date  : 20/04/2018
time  : 00:10:17
*/
#include <bits/stdc++.h>

using namespace std;

#define      pii               std::pair<int,int>
#define      vi                std::vector<int>
#define      sz(v)             (int)(v.size())
#define      mp(a,b)           make_pair(a,b)
#define      pb(a)             push_back(a)
#define      each(it,s)        for(auto it = s.begin(); it != s.end(); ++it)
#define      rep(i, n)         for(int i = 0; i < (n); ++i)
#define      rep1(i, n)        for(int i = 1; i <= (n); ++i)
#define      all(v)            v.begin(), v.end()
#define      scan(n)           scanf("%d", &n)
#define      scan2(n, m)       scanf("%d%d", &n, &m)
#define      pin(n)            printf("%d\n",n)
#define      pis(n)            printf("%d ",n)
#define      pll(n)            printf("%lld\n", n)
#define      X                 first
#define      Y                 second

typedef long long ll;
const ll mod = 1000000007;

inline int pow_(ll a, int n, int p=mod){
int r=1;while(n){if(n&1)r=r*a%p;n>>=1;a=a*a%p;}return r;}
inline int inv_(int a) {return pow_(a, mod-2, mod);}
inline int add(int a, int b){a+=b;if(a>=mod)a-=mod;return a;}
inline void adds(int& a, int b){a+=b;if(a>=mod)a-=mod;}
inline int mul(int a, int b){return a*1ll*b%mod;}
inline void muls(int& a, int b){a=a*1ll*b%mod;}
inline int sub(int a, int b){a-=b;if(a<0)a+=mod;return a;}

const double inf = 1e100;

double getTime(int i, int j, bool dirI, bool dirJ, const vector<pii>& v) {
	if(dirI == 0 && dirJ == 1) return inf;
	if(dirI == 1 && dirJ == 0) {
		double dx = abs(v[i].X - v[j].X);
		double vx = abs(v[i].Y + v[j].Y);
		return dx / vx;
	}
	if((dirI == 0 && v[i].Y >= v[j].Y) || (dirI == 1 && v[i].Y <= v[j].Y)) {
		return inf;
	}
	double dx = abs(v[i].X - v[j].X);
	double vx = abs(v[i].Y - v[j].Y);
	return dx / vx;
}

bool isPossible(double t, const vector<pii> & v) {
	int n = sz(v);
	vector<bool> dir(n, 0);
	for (int i = 1; i < n; ++i) {
		bool curDir = false; //dir[i-1];
		bool f = true;
		for (int j = 0; j < i && f; ++j) {
			double tmpTime = getTime(j, i, dir[j], curDir, v);
			if(tmpTime < t) f = false;
		}
		if(!f) {
			curDir ^= true; f = true;
			for (int j = 0; j < i && f; ++j) {
				double tmpTime = getTime(j, i, dir[j], curDir, v);
				if(tmpTime < t) f = false;
			}
			if(!f) return false;
		}
		dir[i] = curDir;
	}
	return true;
}

int getInt(char endChar) {
	int x = 0;
	char ch = getchar();
	assert(ch >= '0' && ch <= '9');
	while(ch >= '0' && ch <= '9') {
		x = x * 10 + ch - '0';
		ch = getchar();
	}
	assert(ch == endChar);
	return x;
}

int main()
{
	int T = getInt('\n');
	assert(1 <= T && T <= 100);
	while(T--) {
		int n = getInt('\n');
		assert(1 <= n && n <= 50);
		vector<pii> inp(n);
		rep(i, n) {
			int u = getInt(' ');
			int v = getInt('\n');
			assert(0 < u && u <= 1e9);
			assert(0 < v && v <= 1e9);
			inp[i] = mp(u, v);
		}
		sort(all(inp));
		rep(i, n-1) assert(inp[i].X < inp[i+1].X);
		double lo = 0, hi = 1e20, mid = 0;
		for (int iter = 0; iter < 100; ++iter) {
			mid = (hi + lo) * 0.5;
			if(isPossible(mid, inp)) lo = mid;
			else hi = mid;
		}
		double ansT = mid;
		if(ansT >= 1e15) ansT = -1;
		printf("%.10lf\n", ansT);
	}
	return 0;
}