#include <bits/stdc++.h>

#include <utility>

using namespace std;

#define endl "\n"
#define ll long long


void solve() {
  int n, k;
  cin >> n >> k;
  vector<int> a(n);
  for(int i = 0; i < n; i++) {
    cin >> a[i];
  }

  vector<int> upleft(n, 0);
  vector<int> upright(n, 0);

  int top = a[0];
  //first left-to-right
  for(int i = 1; i < n; i++) {
    top -= k;
    upleft[i] = max(top - a[i], 0);
    top = max(top, a[i]);
  }

  // right-to-left
  top = a[n-1];
  for(int i = n-2; i >= 0; i--) {
    top -= k;
    upright[i] = max(top - a[i], 0);
    top = max(top, a[i]);
  }

  int ans = 0;
  for(int i = 0; i < n; i++) {
    ans += max(upleft[i], upright[i]);
  }

  cout << ans << endl;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int t = 1;
  //cin >> t;
  while (t--) {
    solve();
  }
}