import sys
from heapq import heappushpop, heappush
import numpy as np






def main():
    n, k = map(int, input().split())
    A = np.fromstring(input(), np.int64, sep=' ')
    if k == 1:
        print(A[1::2].max())
        exit()
    s = A[-k+1:].sum()
    q = []
    ans = s
    for a in A[-k+1:]: heappush(q, a)
    for i in range(n - k, -1, -1):
        a = A[i]
        if i & 1 and a + s > ans: ans = a + s
        s += a
        s -= heappushpop(q, a)
    print(ans)









if __name__ == '__main__':
    main()