#include <iostream>
#include<stdio.h>
using namespace std;
FILE *f=fopen("deque.in","r");
FILE *g=fopen("deque.out","w");
int inf=10000000,n,k,i,j,s=0,a[5000000],mn;
int main()
{
   fscanf(f,"%d%d",&n,&k);
   for(i=1;i<=n;i++) fscanf(f,"%d",&a[i]);
    i=1;
   while(i<=n-k+1)
   {
       mn=inf;
       for(j=0;j<k;j++)
        if(a[i+j]<mn) mn=a[i+j];
        if(a[i+j]==mn)
        {
            s+=mn;
            if(a[i+j+1]<mn) {mn=a[i+j+1];s+=mn;i+=2;}
            else{s+=mn;i+=2;}
        }
        else
       {
           s+=mn;
           i++;
       }
   }
   fprintf(g,"%d",s);

}