参考：https://blog.csdn.net/FAreStorm/article/details/49200383
没有技术含量但是难想难写，枚举情况图详见参考blog懒得画了
bzoj蜜汁TTTTTTTTTTTTTTTLE
upd：bzoj数据有问题，快读GG
#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
const int N=1505;
int n,m,k,a[N][N],b[N][N],c[N][N],d[N][N],s[N][N],p[N][N],h[N][N],l[N][N],mxh[N],mxl[N],ans;
int read()
{
int r=0,f=1;
char p=getchar();
while(p>'9'||p<'0')
{
if(p=='-')
f=-1;
p=getchar();
}
while(p>='0'&&p<='9')
{
r=r*10+p-48;
p=getchar();
}
return r*f;
}
int main()
{
n=read(),m=read(),k=read();
for(int i=1;i<=n;i++)
for(int j=1;j<=m;j++)
s[i][j]=s[i-1][j]+s[i][j-1]-s[i-1][j-1]+read();
for(int i=k;i<=n;i++)
for(int j=k;j<=m;j++)
p[i][j]=s[i][j]-s[i-k][j]-s[i][j-k]+s[i-k][j-k],a[i][j]=max(p[i][j],max(a[i-1][j],a[i][j-1]));
for(int i=k;i<=n;i++)
for(int j=m-k+1;j>=1;j--)
b[i][j]=max(p[i][j+k-1],max(b[i-1][j],b[i][j+1]));
for(int i=n-k+1;i>=1;i--)
for(int j=k;j<=m;j++)
c[i][j]=max(p[i+k-1][j],max(c[i+1][j],c[i][j-1]));
for(int i=n-k+1;i>=1;i--)
for(int j=m-k+1;j>=1;j--)
d[i][j]=max(p[i+k-1][j+k-1],max(d[i+1][j],d[i][j+1]));
for(int i=1;i<=n;i++)
for(int j=1;j<=m;j++)
mxh[i]=max(mxh[i],p[i][j]),mxl[j]=max(mxl[j],p[i][j]);
for(int i=1;i<=n;i++)
{
h[i][i+k-1]=mxh[i+k-1];
for(int j=i+k;j<=n;j++)
h[i][j]=max(h[i][j-1],mxh[j]);
}
for(int i=1;i<=m;i++)
{
l[i][i+k-1]=mxl[i+k-1];
for(int j=i+k;j<=m;j++)
l[i][j]=max(l[i][j-1],mxl[j]);
}
for(int i=k;i<=n-k+1;i++)
for(int j=k;j<=m-k+1;j++)
ans=max(ans,max(max(a[i][j]+c[i+1][j]+l[j+1][m],l[1][j]+b[i][j+1]+d[i+1][j+1]),max(a[i][j]+b[i][j+1]+h[i+1][n],h[1][i]+c[i+1][j]+d[i+1][j+1])));
for(int i=k;i<=n-k+1;i++)
for(int j=i+k-1;j<=n-k+1;j++)
ans=max(ans,h[1][i-1]+h[i][j]+h[j+1][n]);
for(int i=k;i<=m-k+1;i++)
for(int j=i+k-1;j<=m-k+1;j++)
ans=max(ans,l[1][i-1]+l[i][j]+l[j+1][n]);
printf("%d
",ans);
return 0;
}