N,M=map(int,input().split())
l=[]
inf=float("inf")
dp=[[-inf]*(2) for i in range(N+1)]
for i in range(N):
    tmp=map( int,input().split())
    l.append(sum(tmp))
dp[0][0]=0
for i in range(N):
    for j in range(2):
        dp[i+1][j]=max(dp[i][j],dp[i+1][j])
    dp[i+1][1]=max(dp[i+1][0]+l[i],dp[i+1][1])
    dp[i+1][0]=max(dp[i][1]-l[i],dp[i+1][0])
print(dp[N][1])