import numpy as np
A= list(map(int, input().split()))
N,M=A[0],A[1]
arr= list(map(int, input().split()))
arr=np.array(arr).reshape(N,M)
arr=np.vsplit(arr, N/2)
max_=0
for i in range(int(N/2)):
  arr[i]=np.hsplit(arr[i],M/2)
  for j in range(int(M/2)):
    if max_<np.sum(arr[i][j]):
      max_=np.sum(arr[i][j])
print(max_)