#include<iostream>
#include<string>
#include<map>
#include<algorithm>
using namespace std;
long long S[1515][1515];
long long D[1515][1515];
int main()
{
	int n,m;
	cin>>n>>m;
	int y;
	memset(S,0,sizeof(S));
	memset(D,0,sizeof(D));
	for(int i=1;i<=n;i++)
	{
		for(int v=1;v<=m;v++)
		{
			cin>>y;
			S[i][v]=S[i][v-1]+y;
		}
	}
	for(int i=1;i<=n;i++)
	{
		int max1=-1000000;
		if(i%2==1)
		{
			for(int v=m;v>=2;v--)
			{
				D[i][v]=D[i-1][v-1]+S[i][v];
				if(D[i][v]>max1)
				{
					max1=D[i][v];
				}
				else
				{
					D[i][v]=max1;
				}
			}
		}
		if(i%2==0)
		{
			for(int v=1;v<=m-1;v++)
			{
				D[i][v]=D[i-1][v+1]+S[i][v];
				if(D[i][v]>max1)
				{
					max1=D[i][v];
				}
				else
				{
					D[i][v]=max1;
				}
			}
		}
	}
	if(n==1)
	{
		cout<<max(S[1][1],D[n][2]);
		return 0;
	}
	if(n%2==0)
	{
		cout<<D[n][m-1];
	}
	else
	{
		cout<<D[n][2];
	}

}