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#include <algorithm>
#include <iostream>
#include <vector>
#include <list>
#include <cassert>
using namespace std;

#define FOR(i, n) for(int i = 0, __n = (n); i < __n; i++)

int a[1000];

int main() {
  int n, k;
  scanf("%d%d", &n, &k);

  FOR(i, n) {
    scanf("%d", &a[i]);
  }

  long long res = 0;
  for (int i = 1; i<n; i++) {
    int tgt = a[i-1] - k;
    if (a[i] < tgt) {
      res += tgt - a[i];
      a[i] = tgt;
    }
  }

  for (int i = n - 2; i >= 0; i--) {
    int tgt = a[i+1] - k;
    if (a[i] < tgt) {
      res += tgt - a[i];
      a[i] = tgt;
    }
  }

  printf("%lld\n", res);

  return 0;
}