import sys
import pprint

def main():
    n, k = [int(i) for i in input().split()]
    bears = [int(i) for i in input().split()]

    bears = sorted(bears)

    krumpac = []

    for i in range(0, len(bears)):
        krumpac.append(bears[i] - i * k)

    krumpac = sorted(krumpac)

    #print(krumpac)

    marenka = krumpac[len(krumpac) // 2]
    #print(marenka)

    jemine = 0
    for i in range(len(krumpac)):
        jemine += abs(krumpac[i] - marenka)

    print(jemine)

    
main()