#include "iostream"
    #include "stdio.h"
    #include "algorithm"
    #define II int
    #define LO long long
    #define R register
    #define I 123456
    using namespace std;
    
    
    II t,n;
    
    LO d;
    
    LO a[I], b[I];
    
    LO gg(R LO x) { return x<0 ? -x : x ; }
    
    LO smaler(R LO a,R LO b) { return a<b ? a : b ; }
    
    
    int main()
    {
        scanf("%d",&t);
        while (t--) {
            scanf("%d%lld",&n,&d);
            for(R II i=1;i<=n;i++) scanf("%lld",&a[i]);
            
            if(gg(a[n]-a[1])>d*(n-1)) {
                printf("impossible\n");
                continue ;
            }
            
            b[1]=a[1];
            for(R II i=2;i<=n;i++) b[i]=b[i-1]-d; 
            
            while (b[n]!=a[n]) {
                R LO low=0,up=0,wei,oo=-1e18,op=1e18, ko=0;
                for(R II i=n;i>1;i--)
                {
                    a[i]>b[i] ? low++ : up++ ;     
                    if(a[i]>b[i]) {
                        op=smaler(op,a[i]-b[i]);
                    } 
                    if(low-up>=oo && b[i-1]+d!=b[i]) {
                        oo=low-up;
                        wei=i;
                        ko=op;
                    }
                }
                
                ko=smaler(ko,b[wei-1]+d-b[wei]);
                for(R II i=wei;i<=n;i++) b[i]+=ko;
            }
            
            R LO ans=0;
            for(R II i=1;i<=n;i++) ans+=gg(a[i]-b[i]);
            printf("%lld\n",ans); 
        }
        exit(0);
    }