N, M = map(int, input().split())

dp1 = 0

A = [sum(map(int, input().split())) for i in range(N)]


dp2 = A[0]



for i in A:
    
    dp1, dp2 = max(dp1, dp2 - i), max(dp2, dp1 + i)
    
print(max(dp1, dp2))