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#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define tak cout<<"TAK\n"
#define yes cout<<"YES\n"
#define nie cout<<"NIE\n"
#define no cout<<"NO\n"
using ll = long long;
using ld = long double;
using pll = pair<ll,ll>;
using pdd = pair<ld,ld>;
#define vec vector
#define all(x) (x).begin(), (x).end()
#define sz(x) (x).size()
#define linijki ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define mapa map<ll,ll>
#define umapa unordered_map<ll,ll>
#define se set<ll>
#define ff first
#define ss second
#define str string
const ll alph = 26;
const ll BIN6 = (1LL << 20);
const ll BIN9 = (1LL << 30);
const ll BIN18 = (1LL << 60);
const ll INF5 = 100000;
const ll INF6 = 1000000;
const ll INF9 = 1000000000;
const ll INF18 = 1000000000000000000;
const ld DINF5 = 1e5L;
const ld DINF6 = 1e6L;
const ld DINF9 = 1e9L;
const ll MOD = 1000000007;

ll k,n,m,wyn=0;
vec<ll> v;

void solve(ll l, ll r){
    if(l>=r){
        return;
    }
    ll mx=l;
    for(ll i=l;i<=r;++i){
        if(v[i]>v[mx]){
            mx=i;
        }
    }
    ll w=v[mx], akt=w;
    for (ll i=(mx-1); i>=l;i--) {
        akt-=k;
        if (akt<0){
            break;   
        } 
        if (v[i]< akt) {
            wyn+=akt-v[i];
            v[i]=akt;
        }
    }
    akt=w;
    for (ll i=(mx+1); i<=r;i++) {
        akt-=k;
        if (akt<0){
            break;   
        } 
        if (v[i]< akt) {
            wyn+=akt-v[i];
            v[i]=akt;
        }
    }

    solve(l,(mx - 1));
    solve((mx + 1),r);
}

int main() {
    linijki;
    
    cin>>n>>k;
    v.resize(n);
    for(ll i=0;i<n;++i){
        cin>>v[i];
        //m=max(m,v[i]);
    }
    /*
    for(ll i=0;i<n;++i){
        if(v[i]==m){
            poz_m.pb(i);
        }
    }
    */
    solve(0, n-1);
    cout<<wyn;
    
    
    
    
    return 0;
}