N,M = map(int,input().split())
A = [0 for _ in range(N)]
for i in range(N):
    a = sum(list(map(int,input().split())))
    A[i] = a
A.insert(0,0)
dp = [[0 for _ in range(2)] for _ in range(N+1)]
for i in range(2,N+1):
    dp[i][0] = dp[i-1][1]+A[i-1]
    dp[i][0] = max(dp[i][0],dp[i-1][0])
    dp[i][1] = dp[i-1][0]-A[i-1]
    dp[i][1] = max(dp[i][1],dp[i-1][1])
print(max(dp[N][0],dp[N][1]+A[N]))