#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#define INF 2000000000
#define MaxN 1005
using namespace std;
    
FILE *IN,*OUT;
int N,K,v[MaxN][MaxN],Row[MaxN][MaxN],Column[MaxN][MaxN],Diag[MaxN][MaxN],D[MaxN][MaxN],S=0,Min=INF;

int main()
{
	IN=fopen("ferma2.in","r");
	OUT=fopen("ferma2.out","w");
	
	fscanf(IN,"%d%d",&N,&K);
	for(int i=1;i<=N;i++)
		for(int j=1;j<=i;j++)
		{
			fscanf(IN,"%d",&v[i][j]);
			Row[i][j]=Row[i][j-1]+v[i][j];
			Column[i][j]=Column[i-1][j]+v[i][j];
			Diag[i][j]=Diag[i-1][j-1]+v[i][j];
			S+=v[i][j];
		}
	for(int i=1;i<=N-K;i++)
		for(int j=1;j<=i;j++)
			D[1][1]+=v[i][j];
	for(int i=2;i<=K+1;i++)
		D[i][1]=D[i-1][1]+Row[i+K-2][K-1]-Diag[i+K-3][K-1],Min=min(Min,D[i][1]);
	
	for(int i=2;i<=K+1;i++)
		for(int j=2;j<=i;j++)
			D[i][j]=D[i][j-1]+Column[i-1][j-1]-Column[i+K-2][j-1]-Diag[i-1][j-1]+Diag[i+K-2][j+K-2],Min=min(Min,D[i][j]);
	fprintf(OUT,"%d",S-Min);
	return 0;
}