#include<fstream>
#include<iostream>
#include<deque>
#include<cstdlib>
#define inf 999999
using namespace std;
ifstream in("deque.in");
ofstream out("deque.out");
int n,k,s=0,v[5000000],p=1;
deque <int> q;

 void solve ()
 {

      int min=inf;
         while(!q.empty())
         {
      if( q.back() >= q.front() &&  q.front()<min)
            min=q.front();
          else
            if(q.back()<min)
               min=q.back();
            q.pop_back();
             if(!q.empty())
             q.pop_front();
        }

         s=s+min;

 }

void read ()
{
   in>>n>>k;
   int a;
     for(int i=1; i<=n; ++i)
             in>>v[i];

            int p=1;
               while(p<n-1)
               {
          for(int i=p;  i<k+p; i++)
             q.push_back(v[i]);
                solve();

               p++;
               }



}
int main ()
  {
       read();
       out<<s;
       in.close();
       out.close();
       return 0;
  }