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#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define st first
#define nd second
#define pii pair<ll,ll>
#define pb push_back
#define pf push_front
#define lll __int128
#define si size()
#define int ll
const int A=5e5+3;
bool vis[A];
signed main(){
    ios_base::sync_with_stdio(0);
   cin.tie(0);
    int sum=0,n,a,k;
    cin>>n>>k;
    priority_queue<pii>pq;
    for(int i=1;i<=n;i++){
        cin>>a;
        sum+=a;
        pq.push({a,i});
    }
    while(pq.si){
        pii p=pq.top();
        pq.pop();
        if(vis[p.nd]) continue;
        if(p.nd==0||p.nd==n+1) continue;
        vis[p.nd]=1;
        sum-=p.st;
        pq.push({p.st-k,p.nd-1});
        pq.push({p.st-k,p.nd+1});
    }
    cout<<sum*-1<<'\n';
    return 0; 
}