#include <iostream>
#include <fstream>
#include <deque>
using namespace std;
ifstream fin ("deque.in");
ofstream fout ("deque.out");
int n,k;
long long s,lmin;
deque <long long> v;
void read()
{
    fin >> n >> k;
    for(int i=0;i<n;++i)
    {
        int x;
        fin >> x;
        v.push_back(x);
        if(v.size()==k)
        {

            lmin = min(v[0],v[1]);
            lmin = min(lmin,v[2]);
            v.pop_front();
            s+=lmin;
        }

    }
}
int main()
{
    read();
    fout << s;
    return 0;
}