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#include<bits/stdc++.h>
#define ll long long
#define pb push_back
#define fi first
#define se second
#define lb lower_bound
using namespace std;
int main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	int n; ll k;
	cin>>n>>k;vector<ll>v(n),w(n);
	for(int i=0;i<n;i++){cin>>v[i];w[i]=v[i];}
	for(int i=1;i<n;i++) w[i]=max(w[i],w[i-1]-k);
	for(int i=n-2;i>-1;i--) w[i]=max(w[i],w[i+1]-k);
	ll sum=0;
	for(int i=0;i<n;i++)sum+=w[i]-v[i];
	cout<<sum<<'\n';
	return 0;
}