#include <bits/stdc++.h> 
using namespace std;
const long long mi = -1e7;
int n,m;
int a[1010][1010],f[1010][1010][5];
long long dfs(int x,int y,int fi){
	if(x < 1 || y < 1 || x > n || y > m)return mi;
	if(f[x][y][fi] != mi)return f[x][y][fi];
	if(fi == 0){
		f[x][y][fi] = max(dfs(x+1,y,0),dfs(x+1,y,2)) + a[x][y];
	}
	else if(fi == 1){
		f[x][y][fi] = max(dfs(x-1,y,1),dfs(x-1,y,2)) + a[x][y];
	}
	else{
		f[x][y][fi] = max(max(dfs(x,y-1,0),dfs(x,y-1,1)),dfs(x,y-1,2)) + a[x][y];
	}
	return f[x][y][fi];
}
int main(){
//	freopen("csp2020pj_number.in","r",stdin);
//	freopen("csp2020pj_number.out","w",stdout);
	memset(a,0x3f,sizeof(a));
	scanf("%d%d",&n,&m);
	for(int i = 1;i <= n;i++){
		for(int j = 1;j <= m;j++){
			scanf("%d",&a[i][j]);
			f[i][j][0] = f[i][j][1] = f[i][j][2] = mi;
		} 
	}
	f[1][1][0] = f[1][1][1] = f[1][1][2] = a[1][1];
	cout<<max(dfs(n,m,1),dfs(n,m,2))<<endl;
	
	return 0;
}