.

#include <bits/stdc++.h>
using namespace std;
const long long min_ll=-1e18;
int n,m;
long long w[1010][1010];
long long f[1010][1010][2];
inline long long mx(long long p,long long q,long long r) 
{
    return p>q?(p>r?p:r):(q>r?q:r);
}
long long dfs(int x, int y, int from) 
{
    if(x<1||x>n||y<1||y>m) return min_ll;
    if(f[x][y][from]!=min_ll) return f[x][y][from];
    if(from==0) f[x][y][from]=mx(dfs(x+1,y,0),dfs(x,y-1,0),dfs(x,y-1,1))+w[x][y];
    else f[x][y][from]=mx(dfs(x-1,y,1),dfs(x,y-1,0),dfs(x,y-1,1))+w[x][y];
    return f[x][y][from];
}
int main() 
{
    cin>>n>>m;
    for(int i=1;i<=n;++i) 
    {
        for(int j=1;j<=m;++j) 
        {
            cin>>w[i][j];
            f[i][j][0]=f[i][j][1]=min_ll;
        }
    }
    f[1][1][0]=f[1][1][1]=w[1][1];
    cout<<dfs(n,m,1)<<endl;
    return 0;
}