自认为和抄题解没区别了已经

#include<iostream>
#include<cstdio>
#include<cstring>
#define Re register
#define LL long long 
using namespace std;
LL f[1111][1111][4],a[1111][1111];//[0]=from under  [1]=from left  [2]=from above  [3]=real
inline LL read(){
	register LL x=0,f=1;char ch=getchar();
	while(!isdigit(ch)){if(ch=='-') f=-1;ch=getchar();}
	while(isdigit(ch)){x=(x<<1)+(x<<3)+(ch^48);ch=getchar();}
	return x*f;
}
int main(){
	int n=read(),m=read();
	LL ma=-0x3f3f3f3f3f3f3f3f;
	for(int i=1;i<=n;++i)
		for(int j=1;j<=m;++j)
			a[i][j]=read();
	f[1][1][2] = ma; f[1][1][0] = ma; f[1][1][1] = ma; f[1][1][3] = a[1][1];
	for(int i=2;i<=n;++i){f[i][1][0]=f[i][i][1]=ma;f[i][1][3]=f[i][1][2]=f[i-1][1][3]+a[i][1];}
	for (int i=2;i<=m;++i){f[1][i][0]=f[1][i][2]=ma;}
	for(int i=2;i<=m;++i){
		for(int j=1;j<=n;++j) f[j][i][1]=f[j][i-1][3]+a[j][i];
		for(int j=n-1;j>=1;--j) f[j][i][0]=max(f[j+1][i][1],f[j+1][i][0])+a[j][i];
		for(int j=2;j<=n;++j) f[j][i][2]=max(f[j-1][i][1],f[j-1][i][2])+a[j][i];
		for(int j=1;j<=n;++j) f[j][i][3]=max(max(f[j][i][1],f[j][i][0]),f[j][i][2]);
	}
	printf("%lld",f[n][m][3]);
	return 0;
}