n, m = map(int, raw_input().split())
grid = []
for i in range(n):
    grid.append(map(int, raw_input().split()))

dp = []
for i in range(n):
    dp.append([0]*m)

for i in range(n):
    msl = [max(grid[i][0], 0)]*m
    for j in range(1, m):
        msl[j] = max(msl[j-1]+grid[i][j], 0)

    msr = [max(grid[i][-1], 0)]*m
    for j in range(1, m):
        msr[m-1-j] = max(msr[m-j]+grid[i][m-1-j], 0)

    mslit = [grid[i][0]]*m
    if i>0: mslit[0] += dp[i-1][0]
    dp[i][0] = mslit[0]+msr[1]
    for j in range(1, m):
        top = 0 if i==0 else dp[i-1][j]
        mslit[j] = max(mslit[j-1], top+msl[j-1])+grid[i][j]

        val = 0 if j+2>m else msr[j+1]
        dp[i][j] = mslit[j] + val

    msrit = [grid[i][m-1]]*m
    if i>0: msrit[m-1] += dp[i-1][m-1]
    dp[i][m-1] = max(dp[i][m-1], msrit[m-1]+msl[m-2])
    for j in range(1, m):
        top = 0 if i==0 else dp[i-1][m-1-j]
        msrit[m-1-j] = max(msrit[m-j], top+msr[m-j])+grid[i][m-1-j]

        val = 0 if j+2>m else msl[m-1-j-1]
        dp[i][m-1-j] = max(dp[i][m-1-j], msrit[m-1-j] + val)

print max(dp[-1])