// author : www.codechef.com/users/rajat4064
 
#include<stdio.h>
#include<math.h>
int main(){
    int t;
    scanf("%d",&t);
    while(t--){
        long long int n,i,d;
        scanf("%lli%lli",&n,&d);
        long long int a[n];
        long long int sum=0,avg=0;
        for(i=0;i<n;i++){
            scanf("%lli",&a[i]);
            sum+=a[i];
        }
        if(sum%n!=0){
            printf("-1\n");
            continue;
        }
        avg=sum/n;
        long long int moves1=0;
        for(i=0;i<n-d;i++){
            if(a[i]!=avg){
                long long int x=a[i]-avg;
                a[i]=avg;
                a[i+d]+=x;
                moves1+=abs(x);
                printf("x=%lld abs(x)=%lld llabs(x)=%lld\n", x, abs(x), llabs(x));
            }
        }
        int f=0;
        for(i=0;i<n;i++){
            if(a[i]!=avg){
                f++;
                break;
            }
        }
        if(f!=0){
            printf("-1\n");
            continue;
        }
        else{
            printf("%lli\n",moves1);
        }
    }
return 0;
}

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1
20 2
1000000000 2 1000000000 2 1000000000 2 1000000000 2 1000000000 2 1000000000 2 1000000000 2 1000000000 2 1000000000 2 1000000000 2