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n, k = map(int, input().split())
droga = list(map(int, input().split()))

ile = 0

for i in range(1, n, 1):
    pok = droga[i-1] - k

    if pok > droga[i]:
        ile += pok - droga[i]
        droga[i] += pok - droga[i]

for i in range(n-2, -1, -1):
    pok = droga[i+1] - k

    if pok > droga[i]:
        ile += pok - droga[i]
        droga[i] += pok - droga[i]

print(ile)