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#include <bits/stdc++.h>
#define int ll
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define rng(i,c,n) for(int i=c;i<n;i++)
#define fi first
#define se second
#define vec vector
#define pb push_back
#define sz(a) (int)a.size()
#define all(a) a.begin(),a.end()
typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pii;
void print(){cout<<'\n';}
template<class h,class...t>
void print(const h&v,const t&...u){cout<<v<<' ',print(u...);}

void slv(){
    int n,k;
    cin>>n>>k;

    vi a(n);
    rep(i,n){
        cin>>a[i];
    }

    auto old_a=a;

    rep(i,n){
        if(i and a[i]<a[i-1]-k){
            a[i]=a[i-1]-k;
        }else if(i and a[i-1]<a[i]-k){
            per(j,i){
                if(a[j]<a[j+1]-k){
                    a[j]=a[j+1]-k;
                }
            }
        }
    }

    int ans=0;
    rep(i,n){
        ans+=a[i];
        ans-=old_a[i];
    }
    cout<<ans<<"\n";
}

signed main(){
	ios::sync_with_stdio(0),cin.tie(0);

	int t;
	t=1;
	// cin>>t;

	rep(cs,t){
		slv();
	}
}