PARTITION

Answer is HERE!

#include<bits/stdc++.h>
using namespace std;
int n,m;long long s[2005][2005],f[2005][2005],g[2005][2005];
int main(){
    scanf("%d%d",&n,&m);
    for(int i=1;i<=n;i++){for(int j=1;j<=m;j++)scanf("%lld",&s[i][j]);}
    for(int i=n;i;i--){for(int j=1;j<=m;j++)s[i][j]+=s[i+1][j];}
    long long S=0;
    for(int j=1;j<=m;j++)S+=s[1][j];
    memset(f,-0x3f,sizeof f);
    memset(g,-0x3f,sizeof g);
    f[0][1]=g[0][m+1]=0;
    for(int i=1;i<=n+1;i++){
        for(int j=1;j<=m+1;j++)f[i][j]=max(f[i-1][j],f[i][j-1]+s[i][j-1]);
        for(int j=m+1;j;j--)g[i][j]=max(g[i-1][j],g[i][j+1]+s[i][j]);}
    cout<<S+2*f[n+1][m+1]+g[n+1][1];return 0;}

Thank you! With pleasure.