#include<cstdio>
#include<cstring>
#include<vector>
using namespace std;
struct Tree
{
	double L,R;
	Tree():L(0),R(0){}
};
const int maxn=6;
int n,vis[1<<maxn];
double r,w[maxn],sum[1<<maxn];
vector<Tree> tree[1<<maxn];
void dfs(int subset)
{
	if(vis[subset]) return;
	vis[subset]=true;
	bool have_children=false;
	for(int left=(subset-1)&subset;left;left=(left-1)&subset)
	{
		have_children=true;
		int right=subset^left;
		double d1=sum[right]/sum[subset];
		double d2=sum[left]/sum[subset];
		dfs(left);
		dfs(right);
		for(int i=0;i<tree[left].size();i++)
			for(int j=0;j<tree[right].size();j++)
			{
				Tree t;
				t.L=max(tree[left][i].L+d1,tree[right][j].L-d2);
				t.R=max(tree[right][j].R+d2,tree[left][i].R-d1);
				if(t.L+t.R<r) tree[subset].push_back(t);
			}
	}
	if(!have_children) tree[subset].push_back(Tree());
}
int main()
{
#ifdef local
	freopen("pro.in","r",stdin);
#endif
	int T;
	scanf("%d",&T);
	while(T--)
	{
		scanf("%lf%d",&r,&n);
		for(int i=0;i<n;i++) scanf("%lf",&w[i]);
		for(int i=0;i<(1<<n);i++)
		{
			sum[i]=0;
			tree[i].clear();
			for(int j=0;j<n;j++)
				if(i&(1<<j)) sum[i] += w[j];
		}
		int root=(1<<n)-1;
		memset(vis,0,sizeof(vis));
		dfs(root);
		double ans=-1;
		for(int i=0;i<tree[root].size();i++)
			ans=max(ans,tree[root][i].L+tree[root][i].R);
		if(ans!=-1) printf("%.10lf\n",ans);
		else printf("-1\n");
	}
	return 0;
}