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dlugosc,  kroki = [int(x) for x in input().split()]
ciag = [int(x) for x in input().split()]
wynik = 0
for i in range(1,dlugosc):
    if ciag[i-1]-kroki > ciag[i]:
        wynik += ciag[i-1]-kroki - ciag[i]
        ciag[i] = ciag[i-1]-kroki
    elif ciag[i-1]-kroki < ciag[i]:
        for j in range(i-1,-1,-1):
            if ciag[j+1]- kroki > ciag[j]:
                wynik +=  ciag[j+1]- kroki - ciag[j]
                ciag[j] = ciag[j+1]- kroki
            else:
                break
print(wynik)