#include <bits/stdc++.h>

using namespace std;

const int MAXN = 1007;

int h[MAXN];
int min_h[MAXN];
int min_h2[MAXN];

int main(){
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);

  int n, k;
  long long wynik = 0;
  cin >> n >> k;

  for(int i = 0; i < n; i++){
    cin >> h[i];
    if(i == 0){
      min_h[i] = h[i];
    }else{
      min_h[i] = max(h[i], min_h[i-1]-k);
    }
  }

  for(int i = n; i >= 0; i--){
    if(i == n){
      min_h2[i] = h[i];
    }else{
      min_h2[i] = max(h[i], min_h2[i+1]-k);
    }
    wynik += (max(min_h[i], min_h2[i]) - h[i]);
  }
  cout << wynik << "\n";

  return  0;
}