# coding: utf-8
# Your code here!
N,M=map(int,input().split())
A=[]
for i in range(N):
    temp=list(map(int,input().split()))
    A.append(sum(temp))

dp=[0,0]

for a in A:
    dp[0],dp[1]=max(dp[1]-a,dp[0]),max(dp[0]+a,dp[1])
    
#print(A)
print(max(dp))