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


d = list(map(int, input().split()))

dirt_used = 0

for i in range(1, n):
    diff = d[i - 1] - d[i]

    if diff > 0 and diff > k:
        dirt_to_add = diff - k
        d[i] += dirt_to_add
        dirt_used += dirt_to_add

# print(dirt_used)

for i in range(n - 2, -1, -1):
    diff = d[i + 1] - d[i]

    if diff > 0 and diff > k:
        dirt_to_add = diff - k
        d[i] += dirt_to_add
        dirt_used += dirt_to_add

print(dirt_used)