N, M = map(int, input().split())
A = []
for i in range(N):
    A.append(sum(list(map(int, input().split()))))
dp = [[0, 0] for _ in range(N + 1)]
for i in range(N):
    dp[i + 1][0] = max(dp[i][0], dp[i][1] - A[i])
    dp[i + 1][1] = max(dp[i][0] + A[i], dp[i][1])
print(max(dp[N]))