/*
    JAI JAGANNATH!
*/
//@Author : zanj0
#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
using namespace std;
typedef long long int lli;
#define ff              first
#define ss              second
#define pb              push_back
#define mp              make_pair
#define pii             pair<lli,lli>
#define vi              vector<lli>
#define mii             map<lli,lli>
#define pqb             priority_queue<lli>
#define pqs             priority_queue<lli,vi,greater<lli> >
#define setbits(x)      __builtin_popcountll(x)
#define zrobits(x)      __builtin_ctzll(x)
#define mod             1000000007
#define inf             1e18
#define ps(x,y)         fixed<<setprecision(y)<<x
#define mk(arr,n,type)  type *arr=new type[n];
#define w(x)            lli x; cin>>x; while(x--)
mt19937                 rng(chrono::steady_clock::now().time_since_epoch().count());
#define ONLINE_JUDGE
typedef tree<lli, null_type, less<lli>, rb_tree_tag, tree_order_statistics_node_update> pbds;


void zanj0()
{
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
}
void out(vector < vector <lli> >& v) {
    for (lli i = 0; i < v.size(); i++) {
        for (lli j = 0; j < v[i].size(); j++) {
            cout << v[i][j] << " ";
        }
        cout << endl;
    }
    cout << endl;
}
void solve() {
    lli n, k;
    cin >> n >> k;
    vector < vector <lli> > v(n + 20, vector <lli>(n + 20));

    for (lli i = 1; i <= n; i++) {
        string s;
        cin >> s;
        for (lli j = 1; j <= n; j++) {
            v[i][j] = s[j - 1] == 'B' ? 0 : 1;
        }
    }
    //out(v);
    vector < vector <lli> > prefixTop(v.begin(), v.end());
    for (lli i = 1; i < v.size(); i++) {
        for (lli j = 0; j < v[i].size(); j++) {
            prefixTop[i][j] += prefixTop[i - 1][j];
        }
    }
    vector < vector <lli> > prefixLeft(v.begin(), v.end());
    for (lli i = 0; i < v.size(); i++) {
        for (lli j = 1; j < v[i].size(); j++) {
            prefixLeft[i][j] += prefixLeft[i][j - 1];
        }
    }
    //out(prefixTop);
    //out(prefixLeft);
    vector < vector <lli> > suffixBottom(v.begin(), v.end());
    vector < vector <lli> > suffixRight(v.begin(), v.end());
    for (lli i = v.size() - 2; i >= 0; i--) {
        for (lli j = 0; j < v[i].size(); j++) {
            suffixBottom[i][j] += suffixBottom[i + 1][j];
        }
    }
    for (lli i = v.size() - 1; i >= 0; i--) {
        for (lli j = v[i].size() - 2; j >= 0; j--) {
            suffixRight[i][j] += suffixRight[i][j + 1];
        }
    }
    //out(suffixBottom);
    //out(suffixRight);
    vector < vector <lli> > validCol(n + 20, vector <lli>(n + 20));
    vector < vector <lli> > validRow(n + 20, vector <lli>(n + 20));
    for (lli i = 1; i + k - 1 <= n; i++) {
        for (lli j = n;  j >= 0; j--) {
            lli oneAtTop = prefixTop[i - 1][j];
            lli oneAtBottom = suffixBottom[i + k][j];
            lli total = n;

            validCol[i][j] = ((oneAtBottom + oneAtTop + k) == total) + validCol[i][j + 1];
        }
    }
    //cout << "Okay!"  << endl;
    //out(validCol);
    for (lli i = v.size() - 2; i >= 0; i--) {
        for (lli j = 1; j + k - 1 <= n; j++) {

            lli oneAtLeft = prefixLeft[i][j - 1];
            lli oneAtRight = suffixRight[i][j + k];
            //cout << i << " " << j << " " << oneAtLeft << " " << oneAtRight << " " << k << endl;
            validRow[i][j] = ((oneAtRight + oneAtLeft + k) == n) + validRow[i + 1][j];
        }
    }

    //out(validRow);
    lli ret = 0;
    lli x = -1, y = -1;
    for (lli i = 1; i + k - 1 <= n; i++) {
        for (lli j = 1 ; j + k - 1 <= n; j++) {
            lli temp = validCol[i][j] - validCol[i][j + k];
            lli rows = validRow[i][j] - validRow[i + k][j];
            //cout << i << " " << j << " " << temp << " " << rows << endl;
            lli z = temp + rows;
            if (z > ret) {
                ret = z;
                x = i;
                y = j;
            }
        }
    }
    for (lli i = 1; i <= n; i++) {
        lli temp = 0;
        for (lli j = 1; j <= n; j++) {
            if (i >= x && i <= x + k - 1 && j >= y && j <= y + k - 1) continue;
            temp += v[i][j];
        }
        ret += temp == n;
    }
    cout << ret << endl;
}
int main()
{
    zanj0();
    solve();
    return 0;
}