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n, k = [int(s) for s in input().split()]
ai = [int(s) for s in input().split()]
ile_zwiru = 0
for i in range(1, len(ai)):
    if ai[i - 1] - ai[i] > k:
        ile_zwiru += ai[i - 1] - k - ai[i]
        ai[i] = ai[i - 1] - k
    elif ai[i] - ai[i - 1] > k:
        j = i - 1
        while j >= 0 and ai[j + 1] - ai[j] > k:
            ile_zwiru += ai[j + 1] - k - ai[j]
            ai[j] = ai[j + 1] - k
            j -= 1
print(ile_zwiru)