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n, k = (int(x) for x in input().split())
initial = [int(x) for x in input().split()]
target = initial.copy()
for _ in range(2):
	for i in range(1, n):
		target[i] = max(target[i], target[i-1]-k)
	target.reverse()
print(sum(x-y for x, y in zip(target, initial)))