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

long long int arr[10001],k,n;


bool check(long long int i,long long int start,long long int end,long long int mid)
{
    
    if(mid>0&&end>=n-1&&start<=0)
        {
        return true;
    }
    if(mid<=0)
        {
        return false;
    }
            return check(i+2*k,max(0LL,i-k),min(n-1,i+k),mid-arr[i])||check(i+1,i,i,mid);
    
}


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