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

using namespace std;

vector <long long> V;

int main()
{
    int n, m;
    scanf("%d %d", &n, &m);
    unsigned long long ans = 0;
    long long x;
    for (int i=1; i<=n; i++)
    {
        scanf("%lld", &x);
        V.push_back(x);
    }
    long long med = V[n/2];
    int pos = 0;
    while (pos < n/2)
    {
        ans += (abs(V[pos]-med) * 2);
        pos += m;
    }
    pos = V.size()-1;
    while (pos > n/2)
    {
        ans += (abs(V[pos]-med) * 2);
        pos -= m;
    }
    printf("%lld\n", ans);
    return 0;
}