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def solve(n, k, nums):
    output = 0
    for i in range(n - 1):
        if nums[i] - nums[i + 1] > k:
            output += nums[i] - nums[i + 1] - k
            nums[i + 1] += nums[i] - nums[i + 1] - k

    for i in range(n - 1, 0, -1):
        if nums[i] - nums[i - 1] > k:
            output += abs(nums[i] - nums[i - 1]) - k
            nums[i - 1] += abs(nums[i] - nums[i - 1]) - k

    return output

n, k = list(map(int, input().split()))
nums = list(map(int, input().split()))
print(solve(n, k, nums))