import sys
input=lambda: sys.stdin.readline().rstrip()
n,m=map(int,input().split())
A=[]
for _ in range(n):
	A.append(sum([int(i) for i in input().split()]))
m1,m2=0,-float("inf")
for a in A:
	m1,m2=max(m1,m2-a),max(m1+a,m2)
print(max(m1,m2))