C++ :

#include <stdio.h>
#include <string.h>
#include <iostream>
#include <string>
#include <queue>
#include <stack>
#include <math.h>
#include <map>
#include <vector>
#include <list>
#include <stdlib.h>
#include <set>
#include <algorithm>
using namespace std;

typedef long long ll;
#define rep(i,s,t) for(int i=s;i<t;i++)
#define red(i,s,t) for(int i=s-1;i>=t;i--)
#define ree(i,now) for(int i=head[now];i!=-1;i=edge[i].next)
#define clr(a,v) memset(a,v,sizeof a)
#define MID(x,y) int mid=(x+y)>>1
#define L t<<1
#define R t<<1|1
#define mk(i,j) make_pair(i,j)
#define SQR(x) ((x)*(x))

inline int input(){
    int ret=0;int isN=0;char c=getchar();
    while(c<'0' || c>'9'){
        if(c=='-') isN=1;
        c=getchar();
    }
    while(c>='0' && c<='9'){
        ret=ret*10+c-'0';
        c=getchar();
    }
    return isN?-ret:ret;
}

inline void output(int x){
    if(x<0){
        putchar('-');x=-x;
    }
    int len=0,data[11];
    while(x){
        data[len++]=x%10;x/=10;
    }
    if(!len)    data[len++]=0;
    while(len--)
        putchar(data[len]+48);
    putchar('\n');
}

inline double dis(int a,int b,int w){
    return sqrt(SQR(a-b)+SQR(w));
}
double dp[110][110];
int n,w,a[110];

int main(){
    int t=input();
    while(t--){
        n=input(),w=input();
        rep(i,0,101) rep(j,0,101) dp[i][j]=0.0;
        rep(i,1,n+1) a[i]=input();
        rep(i,1,a[1]+1) dp[1][i]=0.0;
        rep(i,2,n+1){
            rep(j,1,a[i]+1){
                rep(k,1,a[i-1]+1){
                    dp[i][j]=max(dp[i][j],dp[i-1][k]+dis(j,k,w));
                }
            }
        }
        double ans=0;
        rep(i,1,a[n]+1) ans=max(ans,dp[n][i]);
        printf("%.3lf\n",ans);
    }
    return 0;
}