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


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    int n,k,cost=0,mid;
    cin>>n>>k;
    vector <int> arr;
    unsigned long long int temp;
    for(int i=0;i<n;i++)
        {
        cin>>temp;
        arr.push_back(temp);
    }
    if(n%2!=0)
        mid=(n/2)+1;
    else
        mid=n/2;
    int low,high;
    if(k!=0){
    low=mid-k;
    high=mid+k;
    }
    else
        {
        low=1;
        high=n;
    }
    for(int i=low-1;i<high;i++)
        {
        if(i==(mid-1))
            {}
        else
        arr[i]=0;
    }
    for(int i=0;i<n;i++)
    {cost+=arr[i];
    //cout<<arr[i];
    }
    cout<<cost<<endl;
    return 0;
}