#pragma region
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
// File
ifstream fin("deque.in");
ofstream fout("deque.out");
#pragma endregion

int main() {
    ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    int n, k;
    fin >> n >> k;

    vector<int> myDeque;
    int front = 0, back = -1;
    ll sum = 0;
    for (int i = 1; i <= n; i++) {
        int x;  fin >> x;
        myDeque.push_back(x);
        back++;
        if (back - front == k - 1) {
            sum += min(min(myDeque[front], myDeque[front + 1]), myDeque[back]);
            front++;
        }
    }
    fout << sum;

    return 0;
}