import sys
input = sys.stdin.readline

T=int(input())

for tests in range(T):
    n,k=map(int,input().split())
    A=list(map(int,input().split()))

    A2=[]
    for a in A:
        if a!=0:
            A2.append(a)

    A=A2
    n=len(A)

    if n==0:
        print(0)
        continue

    if n<k:
        print(-1)
        continue

    B=[a%k for a in A]
    B.sort()

    SUM=sum(B)

    if SUM%k!=0:
        print(-1)
        continue

    if n==k:
        x=B[0]
        flag=1

        for i in range(n):
            if x!=B[i]:
                flag=0
                break

        if flag:
            print(x)
        else:
            print(-1)
        continue

    X=[]
    for i in range(n):
        if B[i]!=0:
            X.append(B[i])

    if len(X)==0:
        print(0)
        continue

    if len(X)==k:
        x=X[0]
        flag=1

        for i in range(len(X)):
            if x!=X[i]:
                flag=0
                break

        if flag:
            print(x)
            continue

    while len(X)<k+1:
        X.append(k)

    print(sum(X)//k)