#include <iostream>
#include <algorithm>
#include <vector>

#define ll long long

using namespace std;

int main() {
	ll n,k; cin>>n>>k; vector<ll> a(n);
	for(ll i=0; i<n; i++) {cin>>a[i];}
	sort(a.begin(),a.end());
	for(ll i=0; i<n; i++) {a[i]-= i*k;}
	ll med = n/2;
	ll sum = 0;
	for(ll i=0; i<n; i++) sum+=abs(a[i]-a[med]);
	cout<<sum<<endl;
	return 0;
}