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inputt = input().split()
n, k = inputt
n = int(n)
k = int(k)
zwir = [0 for _ in range(n)]
inputt = input().split()
for i in range(n):
    zwir[i] = int(inputt[i])
ciezarowki = 0
for i in range(1,n):
    if zwir[i] < zwir[i-1] - k:
        ciezarowki += zwir[i-1] - k - zwir[i]
        zwir[i] = zwir[i-1] - k
for j in range(n-2, -1, -1):
    if zwir[j] < zwir [j+1] - k:
        ciezarowki += zwir[j+1] - k - zwir[j]
        zwir[j] = zwir[j+1] - k
print(ciezarowki)