杭电1969 pie

#include<bits/stdc++.h>
#define PI acos(-1.0)
#define eps 1e-7
#define inf 0x3f3f3f3f
using namespace std;

double a[10004];
double mid;
int num;
int n,m;
double maxn;
double minn;
int main(){
	int t;
	cin>>t;
	while(t--){
		cin>>n>>m;
		m = m+1;
	    maxn = -1;
		for(int i=0;i<n;i++){
			cin>>a[i];
			a[i] = a[i]*a[i];
			if(a[i]>maxn) maxn = a[i];
		}
	    minn = 0;
		while(maxn - minn > eps){
			mid = (maxn+minn)/2;
			num = 0;
			for(int i=0;i<n;i++){
				num += (int)(a[i]/mid);
			}
			if(num >= m) minn = mid;
			else  maxn = mid;
		}
		printf("%.4lf\n",mid*PI);
	}
	return 0;
}