#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;
int K,N,tinggi,A[1000000];

int maksimum(int p)
{
	int gg = 0;
	for (int f=p;f<=(p+K-1);f++)
	{
		gg = max(A[f],gg);
	}
	return gg;
}

int main()
{
	scanf("%d%d",&N,&K);
	for (int i=1;i<=N;i++)
	{
		scanf("%d",&A[i]);
	}
	if ((N%K)==0)
	{
		tinggi = 1 + (N/K);
	} else
	{
		tinggi = 2 + (N/K);
	}
	for (int i=1;i<=N;i+=K)
	{
		tinggi = tinggi + maksimum(i);
	}
	printf("%d\n",tinggi);
}