int n,m;
int dp[1010][1010][3];
int a[1010][1010];
int was[1010][1010][3];
int r(int x, int y, int from)
{
    if ((x==n-1) && (y==m-1))
        return a[x][y];
    if (was[x][y][from])
        return dp[x][y][from];
    was[x][y][from]=1;
    int res=-1000000000;
    if (x<n-1)
        res=max(res,a[x][y]+r(x+1,y,0));
    if ((y<m-1) && (from!=2))
        res=max(res,a[x][y]+r(x,y+1,1));
    if ((y>=0) && (from!=1))
        res=max(res,a[x][y]+r(x,y-1,2));
    return dp[x][y][from]=res;
}
int main()
{
    scanf("%d%d",&n,&m);
    FOR(i,0,n)
        FOR(j,0,m)
            scanf("%d",&a[i][j]);
    int res=r(0,0,0);
    cout<<res<<endl;
    return 0;
}