#include <fstream>
#include <deque>

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

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

 static inline int MIN3(int a, int b, int c)
{
    int small=a;
    if (small>b) small=b;
    if (small>c) small=c;
    return small;
}

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;
}