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n, k = map(int,input().split())
a = list(map(int,input().split()))
#print(a)

ans = 0
for i in range(1,n):
	r = a[i-1]-a[i]
	if r>k:
		a[i] += r-k
		ans += r-k
for i in range(n-2,-1,-1):
	r = a[i+1]-a[i]
	if r>k:
		a[i] += r-k
		ans += r-k
#print(a)
print(ans)