#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <iterator>
using namespace std;

int main() {
  int n, k;
  cin >> n >> k;
  long long mins[2 * k + 1];
  for(int i = 0; i < n; ++i) {
    if(i < 2 * k + 1) cin >> mins[i];
    else {
      long long next;
      cin >> next;
      mins[(i % (2 * k + 1))] += next;
    }
  }
  cout << *min_element(mins, mins + min(n, k + 1)) << endl;
  return 0;
}