#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>

int main() {

    /* Enter your code here. Read input from STDIN. Print output to STDOUT */  
    //Find out what total overlap is required at the edges
    //Then compare the costs of using each overlap
    
    int arraySize=0;
    int range=0;
    int scale=0;
    int i=0;
    int overlap=0;
    int found=0;
    int first=0;
    int more=1;
    long long int count=1;
    
    int startPoint=0;
    
    unsigned long long int* costArray;
    
    unsigned long long int totalCost=0;
    unsigned long long int totalCost2=0;
    unsigned long long int lowestCost=0;
    
    scanf("%d %d", &arraySize, &range);
    
    scale=1+(2*range);
    
    costArray=malloc(arraySize*sizeof(unsigned long long int));
    
    for(i=0;i<arraySize;i++)
    {
        scanf("%llu", &costArray[i]);
    }
    
    if(arraySize>scale)
    {
    
        while(found==0)
        {
            if((arraySize-overlap)%scale==0)
            {
                found=1;
            }
            else
            {
                overlap++;
            }
        }
        
        //Test overlap at one end, then the other
        
        
        
        startPoint=range+1-overlap;
        if(startPoint<0)
        {
            startPoint=0;
        }
        totalCost=costArray[startPoint];
        count=startPoint+scale;
        
        while(more==1)
        {
            totalCost+=costArray[count];
            count+=scale;
            if(count>arraySize)
            {
                more=0;
            }
        }
        
        more=1;
        totalCost2=costArray[arraySize-startPoint-1];
        count=arraySize-startPoint-1-scale;
        
        while(more==1)
        {
            totalCost2+=costArray[count];
            count-=scale;
            if(count<0)
            {
                more=0;
            }
        }
        
        if(totalCost>totalCost2)
        {
            lowestCost=totalCost2;
        }
        else
        {
            lowestCost=totalCost;
        }
        
    }
    else
    {
        //find minimum cost
        lowestCost=999999999999;
        for(i=0;i<arraySize;i++)
        {
            if(costArray[i]<lowestCost)
            {
                lowestCost=costArray[i];
            }
        }
    }
    
    printf("%llu\n", lowestCost);
    
    return 0;
}