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

dp = [[0] * 2 for _ in range(N)]
dp[0][1] = B[0]
for i in range(1,N):
    dp[i][0] = max(dp[i-1][1] - B[i],dp[i-1][0])
    dp[i][1] = max(dp[i-1][0] + B[i],dp[i-1][1])
print(max(dp[-1]))