#include <bits/stdc++.h>
using namespace std;
int main() {
    long long n,k;
    //freopen("FOOD.INP","r",stdin);
    //freopen("FOOD.OUT","w",stdout);
    cin>>n>>k;
    vector<long long> a(n);
    for (long long i=0;i<n;i++) 
      cin>>a[i];
    long long t=0;
    priority_queue<pair<long long,long long>,vector<pair<long long,long long>>,greater<pair<long long,long long>>> b;
    for (long long i=0;i<n;i++) {
        b.push({a[i],i});
        while (b.top().second<=i-k) {
            b.pop();
        }
        t+=2*b.top().first;
    }
    cout<<t;
    return 0;
}