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

#define IO ios_base::sync_with_stdio(false); cin.tie(0);
#define endl "\n"
#define DBG 666
#define mp make_pair
#define revsort(x,y) sort((x),(x+y));reverse((x),(x+y));

typedef long long ll;

int main(){

	IO;

	int n, k;
	cin >> n >> k;
	ll cost[n+5];
	for(int i = 0; i < n; i++){
		cin >> cost[i];
	}

	ll res = 0;
	int i = 0;
	while(i < n){

		ll minn = LONG_LONG_MAX;
		for(int j = 0; j <= k && i < n; j++, i++){
			minn = min(minn, cost[i]);
		}
		i += k;
		res += minn;
	}

	cout << res << endl;

	return 0;
}