#include <iostream>
#include <cstdio>
using namespace std;
typedef long long ll;
const int N=5005;
int n,d,de[N];
void clear(){
	return;
}
void work(){
	scanf("%d %d",&n,&d);
	for(int i=1;i<=n;i++)scanf("%d",de+i);
	if(n==1)printf("0\n");
	else if(abs(de[1]-de[2])>d){
		printf("-1\n");
	}else{
		ll ans=0;
		for(int i=2;i<n;i++){
			ans+=max(abs(de[i]-de[i+1])-d,0);
		}
		printf("%lld\n",ans);
	}
	return;
}
int main(){
	freopen("tiger.in","r",stdin);
	freopen("tiger.out","w",stdout);
	int T;
	scanf("%d",&T);
	while(T--){
		work();
		clear();
	}
	return 0;
	
}