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#!/usr/bin/env python3

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

A_final = [x for x in A_init]
for i in range(n - 1):
    A_final[i + 1] = max(A_final[i + 1], A_final[i] - k)
for i in range(n - 1, 0, -1):
    A_final[i - 1] = max(A_final[i - 1], A_final[i] - k)

print(sum(A_final) - sum(A_init))