#include <fstream>
///7
using namespace std;
ifstream fin("deque.in");
ofstream fout("deque.out");
int  Minim(int a, int b,int c)
{
    if(a<b && a<c) return a;
      if(b<c && b<a) return b;
       if(c<b && c<a) return c;
}
int main()
{
    int i,n,k,v[1001];
    fin>>n>>k;
    int s=0;
    for(i=1;i<=n;i++) fin>>v[i];
    for(i=1;i<=n-k+1;i++)
    s=s+Minim(v[i],v[i+1],v[i+2]);
    fout<<s;

    return 0;
}