#include <iostream>
#include <fstream>
#include <algorithm>

using namespace std;

int main() {
    ifstream fin("deque.in");
    ofstream fout("deque.out");

    int N, K;
    fin>>N>>K;
    long long v[N + 1];
    long long suma = 0, aux;
    for(int i = 0; i < N; i++){
        long long x;
        fin>>x;
        v[i] = x;
    }

    for(int j = 0; j < N - K + 1; j++){
        aux = min(v[j], v[j + 1]);
        suma += min(aux,  v[j + 2]);
    }

    fout<<suma;

    fin.close();
    fout.close();
    return 0;
}