#include <bits/stdc++.h>
using namespace std;
#define EPS (1e-10)
#define REP (i,n) for (int i = 0; i<(n);i++)

typedef long double ld;
typedef long long ll;
typedef pair<int,int> ii;
typedef pair<double, double> dd;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<double> vd;

int main() {
    ios::sync_with_stdio(false);
    int N, a; cin >> N >> a;
    vi V(N);
    for(int i = 0; i < N; ++i) cin >> V[i];
    sort(V.begin(), V.end());
    
    // while(lb <= ub) {
    //     int mid = (lb+ub)/2;
    //     ll c1 = 0, c2 = 0;
    //     for(int i = 0; i < N; ++i) {
    //         if (V[i] > mid + i*a) c1++;
    //         else if (V[i] < mid + i*a) c2++;
    //     }
    //     if (c1 == c2) {
    //         res = mid;
    //         break;
    //     }  else if (c1 < c2) {
    //         ub = mid-1;
    //     } else {
    //         lb = mid+1;
    //     }
    // }
    // ll ss = 0;
    // // cout << res << endl;
    int lb = V[0], ub = V[N-1];
    while (ub-lb >= 3) {
        int aa = lb+(ub-lb)/3;
        int ab = ub-(ub-lb)/3;
        long long s1=0, s2=0;
        for(int i = 0; i < N; ++i) {
            s1 += abs(V[i] - aa-i*a);
        }
        for(int i = 0; i < N; ++i) {
            s2 += abs(V[i] - ab-i*a);
        }
        if (s1 > s2) lb = aa;
        else ub = ab;
        
    }
    // cout << lb << " " << ub << endl;
    ll res = LLONG_MAX;
    for(int r = lb-3; r <= ub+3; ++r) {
    ll s2 = 0;
    for(int i = 0; i < N; ++i) {
            s2 += abs(V[i] - r-i*a);
        }
        res = min(s2, res);
    }
    cout << res << endl;

}