// Codeforces 1200D - White Lines
// Lúcio Cardoso

#include <bits/stdc++.h>

using namespace std;

const int maxn = 2e3+10;

char a[maxn][maxn];

int linha[maxn][maxn], col[maxn][maxn];
int prefL[maxn][maxn], prefC[maxn][maxn];

int firstL[maxn], lastL[maxn];
int firstC[maxn], lastC[maxn];

int totL[maxn], totC[maxn];

int main(void)
{
	int n, k;
	scanf("%d %d", &n, &k);

	for (int i = 1; i <= n; i++)
	{
		for (int j = 1; j <= n; j++)
			scanf(" %c", &a[i][j]);
	}

	for (int i = 1; i <= n; i++)
	{
		for (int j = 1; j <= n; j++)
		{
			if (a[i][j] == 'B')
			{
				if (firstL[i] == 0) firstL[i] = j;
				if (firstC[j] == 0) firstC[j] = i;

				lastC[j] = i;
			}
		}

		for (int j = n; j >= 1; j--)
			if (a[i][j] == 'B')
				if (lastL[i] == 0) lastL[i] = j;
	}

	for (int i = 1; i <= n; i++)
	{
		for (int j = 1; j <= n; j++)
		{
			if (firstL[i] >= j && lastL[i]-j < k)
				linha[i][j]++;
			if (!firstL[i])
				linha[i][j]++;

			if (firstC[j] && firstC[j] >= i && lastC[j]-i < k)
				col[i][j]++;
			if (!firstC[j])
				col[i][j]++;
		}
	}

	for (int i = 1; i <= n; i++)
	{
		for (int j = 1; j <= n; j++)
		{
			prefL[i][j] = prefL[i][j-1] + col[i][j];
			prefC[i][j] = prefC[i-1][j] + linha[i][j];
		}
	}

	for (int i = 1; i <= n; i++)
	{
		bool ok = 1;
		for (int j = 1; j <= n; j++)
			if (a[i][j] == 'B')
				ok = 0;

		if (ok) totL[i] = totL[i-1]+1;
		else totL[i] = totL[i-1];
	}

	for (int i = 1; i <= n; i++)
	{
		bool ok = 1;
		for (int j = 1; j <= n; j++)
			if (a[j][i] == 'B')
				ok = 0;

		if (ok) totC[i] = totC[i-1]+1;
		else totC[i] = totC[i-1];
	}

	int ans = 0;

	for (int i = 1; i+k-1 <= n; i++)
		for (int j = 1; j+k-1 <= n; j++)
			ans = max(ans, prefL[i][j+k-1]-prefL[i][j-1] + prefC[i+k-1][j]-prefC[i-1][j] + totL[n]-(totL[i+k-1]-totL[i-1]) + totC[n]-(totC[j+k-1]-totC[j-1]));

	printf("%d\n", ans);
}