n, k = map(int,input().strip().split())

l = list(map(int,input().strip().split()))

c = 0

while len(l) > 0:
    if len(l) > 2*k+1:
        if l[k] <= l[-k-1] :
            c += l[k]
            i = 2*k
            while i != -1:
                del l[i]
                i-=1
        else:
            c += l[-k-1]
            i = -2*k-1
            while i != 0:
                del l[i]
                i+=1
            
    elif len(l) < 2*k+1:
        c+=min(l)
        l.clear()
    else:
        c +=l[k]
        l.clear()
        
        
print(str(c))