N,M=map(int,input().split())
B=[0]*N
for i in range(N):
    A=list(map(int,input().split()))
    B[i]=sum(A)

DP=[[0,0] for _ in range(N+1)]

for x in range(1,N+1):
    DP[x][0]=max(DP[x-1][0],DP[x-1][1]+B[x-1])
    DP[x][1]=max(DP[x-1][0]-B[x-1],DP[x-1][1])

print(DP[-1][0])