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n, k = input().split()
path = [int(x) for x in input().split()]
count = 0

for i in range(0, len(path) - 1):
    if path[i] > path[i + 1]:
        diff = path[i] - path[i + 1]
        if diff > int(k):
            add_level = diff - int(k)
            count += add_level
            path[i + 1] += add_level

for i in range(len(path) - 1, 0, -1):
    if path[i] > path[i - 1]:
        diff = path[i] - path[i - 1]
        if diff > int(k):
            add_level = diff - int(k)
            count += add_level
            path[i - 1] += add_level

print(count)