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#include <cstdio>

int A[1000];

int main(void) {
  int n, k, s = 0;
  scanf("%d%d", &n, &k);
  for (int i = 0;   i < n;  i++) scanf("%d", &A[i]);
  for (int i = 1;   i < n;  i++) if (A[i] < A[i-1] - k) s += A[i-1] - k - A[i], A[i] = A[i-1] - k;
  for (int i = n-2; i >= 0; i--) if (A[i] < A[i+1] - k) s += A[i+1] - k - A[i], A[i] = A[i+1] - k;
  printf("%d\n", s);
}