n, m = map(int, input().split())
a = [sum(list(map(int, input().split()))) for i in range(n)]
dp = [[0] * 2 for i in range(n + 1)]
for i in range(1, n + 1):
    dp[i][0] = max(dp[i - 1][0], dp[i][1] - a[i - 1])
    dp[i][1] = max(dp[i - 1][1], dp[i][0] + a[i - 1])
print(dp[-1][-1])