#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int main() {
	cin.tie(nullptr)->sync_with_stdio(false);
	freopen("deque.in", "r", stdin);
	freopen("deque.out", "w", stdout);
	int n, k;
	cin >> n >> k;
	vector<int> a(n), prf(n), suf(n);
	for (auto &it : a) {
		cin >> it;
	}
	for (int i = 0; i < n; i++) {
		if (i % k == 0) {
			prf[i] = a[i];
		} else {
			prf[i] = min(prf[i - 1], a[i]);
		}
	}
	for (int i = n - 1; i >= 0; i--) {
		if (i % k == 0 || i == n - 1) {
			suf[i] = a[i];
		} else {
			suf[i] = min(suf[i + 1], a[i]);
		}
	}
	ll ans = 0;
	for (int i = 0; i + k - 1 < n; i++) {
		ans += min(suf[i], prf[i + k - 1]);
	}
	cout << ans;
	return 0;
}