#include <cstdio>

 #include <cstring>

 #include <algorithm>

 #include <vector>

 #include <map>

 #include <cmath>

 #define MP make_pair

 #define Ft first

 #define Sd second

 using namespace std;

 typedef pair<double, double> PDD;

 const int maxn = ;

 const int maxs = ;

 vector<PDD> tree[maxs];

 int n;

 double r;

 double a[maxn], w[maxs];

 bool vis[maxs];

 int bitcount(int x)

 {

     int ans = ;

     while(x) { ans += (x & ); x >>= ; }

     return ans;

 }

 void dfs(int S)

 {

     if(vis[S]) return ;

     vis[S] = true;

     if(bitcount(S) == ) { tree[S].push_back(MP(, )); return ; }

     PDD t = MP(, );

     for(int s1 = (S-)&S; s1; s1 = (s1-)&S)

     {

         int s2 = S ^ s1;

         dfs(s1); dfs(s2);

         double x1 = w[s2] / w[S], x2 = w[s1] / w[S];

         for(int i = ; i < tree[s1].size(); i++)

             for(int j = ; j < tree[s2].size(); j++)

             {

                 t.Ft = max(x1 + tree[s1][i].Ft, tree[s2][j].Ft - x2);

                 t.Sd = max(x2 + tree[s2][j].Sd, tree[s1][i].Sd - x1);

                 if(t.Ft + t.Sd < r) tree[S].push_back(t);

             }

     }

 }

 int main()

 {

     int T; scanf("%d", &T);

     while(T--)

     {

         scanf("%lf%d", &r, &n);

         for(int i = ; i < n; i++) scanf("%lf", a + i);

         int all = ( << n) - ;

         for(int i = ; i <= all; i++)

         {

             w[i] = ;

             tree[i].clear();

             for(int j = ; j < n; j++) if(i & ( << j))

                 w[i] += a[j];

         }

         memset(vis, false, sizeof(vis));

         dfs(all);

         double ans = -;

         for(int i = ; i < tree[all].size(); i++) ans = max(ans, tree[all][i].Ft + tree[all][i].Sd);

         if(ans < ) puts("-1");

         else printf("%.9f\n", ans);

     }

     return ;

 }