#include <fstream>
#include <deque>

using namespace std;
ifstream cin("deque.in");
ofstream cout("deque.out");

int N; unsigned int K; long long int Rezult ,Aux;
deque<int> S;

static inline int MIN3(int a, int b, int c)
{
    if (a < b)
    {
        if (a < c)
             return a;
        else
             return c;
    }

    if (b < c)
        return b;
    else return c;
}

int main()
{
    cin>>N>>K;

    while(cin>>Aux) S.push_back(Aux);

    while(S.size()>=K)
    {
        Rezult+=MIN3(S[0],S[1],S[2]);
        S.pop_front();
    }
    cout<<Rezult;
}