# Enter your code here. Read input from STDIN. Print output to STDOUT

n,k = map(int,raw_input().split())
c = map(int,raw_input().split())
m = 10**9
for j in xrange(k+1):
    a = 0
    for i in xrange(j,n,2*k+1):
        a += c[i]
    m = min(m,a)
print m