#include <bits/stdc++.h>
     
    using namespace std;
     
    const int MaxN = (int)2.5e5 + 10;
    const int INF = 1e9;
     
    int n, en;
    int x[MaxN], h[MaxN];
    int b[MaxN], sz;
    long long fenw[2][MaxN];
     
    long long get(long long fenw[], int x) {
    	long long res = 0;
    	while (x > 0) {
    		res += fenw[x];
    		x &= x + 1, --x;
    	}
    	return res;
    }
     
    void upd(long long fenw[], int x, long long val) {
    	while (x <= n) {
    		fenw[x] += val;
    		x |= x + 1;
    	}
    }
     
    int fnd(int x) {
    	return lower_bound(b, b + sz, x) - b;
    }
     
     long long readInt(long long l,long long r,char endd){
	long long x=0;
	int cnt=0;
	int fi=-1;
	bool is_neg=false;
	while(true){
		char g=getchar();
		if(g=='-'){
			assert(fi==-1);
			is_neg=true;
			continue;
		}
		if('0'<=g && g<='9'){
			x*=10;
			x+=g-'0';
			if(cnt==0){
				fi=g-'0';
			}
			cnt++;
			assert(fi!=0 || cnt==1);
			assert(fi!=0 || is_neg==false);
			
			assert(!(cnt>19 || ( cnt==19 && fi>1) ));
		} else if(g==endd){
			if(is_neg){
				x= -x;
			}
			assert(l<=x && x<=r);
			return x;
		} else {
			assert(false);
		}
	}
}
string readString(int l,int r,char endd){
	string ret="";
	int cnt=0;
	while(true){
		char g=getchar();
		assert(g!=-1);
		if(g==endd){
			break;
		}
		cnt++;
		ret+=g;
	}
	assert(l<=cnt && cnt<=r);
	return ret;
}
long long readIntSp(long long l,long long r){
	return readInt(l,r,' ');
}
long long readIntLn(long long l,long long r){
	return readInt(l,r,'\n');
}
string readStringLn(int l,int r){
	return readString(l,r,'\n');
}
string readStringSp(int l,int r){
	return readString(l,r,' ');
}
 
     
    void solve() {
		n = readIntLn(1, 250000);
    	assert (1 <= n && n <= 250000);
    	en += n;
    	assert (1 <= en && en <= 500000);
    	sz = 0;
    	map < int, vector < pair < int, int > > > all;
    	memset(fenw, 0, sizeof(fenw));
    	for (int i = 1; i <= n; ++i) {
			x[i] = readIntSp(1, INF);
			h[i] = readIntLn(1, INF);
    		assert (x[i] <= INF && x[i] >= 1);
    		assert (h[i] <= INF && h[i] >= 1);
    		b[sz++] = x[i];
    		all[x[i] - h[i]].push_back(make_pair(x[i], h[i]));
    	}
    	sort(b, b + sz);
    	sz = unique(b, b + sz) - b;
    	assert (sz == n);
    	double ans = -1e+40;
    	for (map < int, vector < pair < int, int > > > :: iterator it = all.begin(); it != all.end(); ++it) {
    		vector < pair < int, int > > &v = it->second;
    		sort(v.begin(), v.end());
    		double have = 0;
    		for (int i = 0; i < (int)v.size(); ++i) {
    			if (i + 1 < (int)v.size()) {
					assert (v[i].first < v[i + 1].first && v[i].second < v[i + 1].second);
    				int a = fnd(v[i].first), b = fnd(v[i + 1].first);
    				double lab = hypotl(v[i].first - v[i + 1].first + .0,
    									v[i].second - v[i + 1].second + .0);
    				long long w = get(fenw[1], b) - get(fenw[1], a);
    				w += (get(fenw[0], b) - get(fenw[0], a)) * it->first;
    				have += lab - w;
    				ans = max(ans, have);
    				have = max(have, 0.0);	
    			}			
    			upd(fenw[0], fnd(v[i].first), +1);
    			upd(fenw[1], fnd(v[i].first), -it->first);
    		}
    	}
    	if (ans == -1e+40) {
    		printf("-1\n");
    	} else {
    		printf("%.9lf\n", ans);
    	}
    }
    
    
    
    int main() {
    //	freopen("input.txt", "r", stdin);
    	int t = readIntLn(1, 100);
    	while (t --> 0) {
    		solve();
    	}
		assert(getchar()==-1);
    	return 0;
    }