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#include <bits/stdc++.h>
using namespace std;

//typy
using ll = long long;
using pll = pair <long long, long long>;
using pii = pair <int, int>;
using ull = unsigned long long;


//vectory
#define all(_) (_).begin(), (_).end()
#define vec vector
using vl = vector <ll>;
using vi = vector <int>;
using vs = vector <string>;
using vc = vector <char>;
using vpll = vector <pair <long long, long long> >;
using vpii = vector <pair <int, int> >;
using vvint = vector<vector<int>>;
using vvll = vector<vector<long long>>;
using vb = vector <bool>;
using ld = long double;

//mapy, sety
using mll = map <long long, long long>;
using mii = map <int, int>;
using mss = map <string, string>;
using mcc = map <char, char>;
using sl = set <long long>;
using si = set <int>;
using ss = set <string>;
using sc = set <char>;

//unordered mapy, sety
using umll = unordered_map <long long, long long>;
using umii = unordered_map <int, int>;
using umss = unordered_map <string, string>;
using umcc = unordered_map <char, char>;
using usl = unordered_set <long long>;
using usi = unordered_set <int>;
using uss = unordered_set <string>;
using usc = unordered_set <char>;

//tak, nie
#define YES cout<<"YES\n"
#define NO cout<<"NO\n"
#define TAK cout<<"TAK\n"
#define NIE cout<<"NIE\n"
#define Yes cout<<"Yes\n"
#define No cout<<"No\n"
#define Tak cout<<"Tak\n"
#define Nie cout<<"Nie\n"
#define yes cout<<"yes\n"
#define no cout<<"no\n"
#define tak cout<<"tak\n"
#define nie cout<<"nie\n"

//inne
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define LINIJKI ios_base::sync_with_stdio(0);cin.tie(0);
#define sorall(x) sort(x.begin(), x.end());
#define sorcmp(x, cmp) sort(x.begin(), x.end(), cmp);
#define rep(i,a,b) for (int i = (a); i < (b); i++)
#define per(i,a,b) for (int i = (b)-1; i >= (a); i--)
#define nl '\n'
#define elif else if
#define vecout(v) {for(auto _:(v)) cout << _ << ' '; cout << "\n";}
#define wczyt(v) for(auto &_:(v)) cin >> (_);
//-----------------------------------------------------
const bool db = false;

int main(){
    LINIJKI
    ll n, k, m=-1, wyn=0;
    cin >> n >> k;
    if(n == 1){
        cout << 0;
        return 0;
    }
    vl l(n);
    wczyt(l);
    for(ll i = 0; i < n; i++){
        m -= k;
        m = max(m, l[i]);
        if(l[i] < m){
            wyn += m-l[i];
            l[i] = m;
        }
    }
    m = -1;
    for(ll i = n-1; i>= 0; i--){
        m -= k;
        m = max(m, l[i]);
        if(l[i] < m){
            wyn += m-l[i];
            l[i] = m;
        }
    }
    cout << wyn;
}