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#!/usr/bin/env python3

import sys


def main():
    N, K = [int(x) for x in input().split()]
    A = [int(a) for a in input().split()]
    result = 0
    for i in range(1, N):
        di = A[i] - A[i - 1]
        if -di > K:
            diff = -di - K
            A[i] += diff
            result += diff
        elif di > K:
            for j in range(i, 0, -1):
                dj = A[j] - A[j - 1]
                if dj <= K:
                    break
                diff = dj - K
                A[j - 1] += diff
                result += diff
    print(result)


if __name__ == "__main__":
    if len(sys.argv) == 2:
        sys.stdin = open(sys.argv[1])
    main()