#include <bits/stdc++.h>
template <typename T>
void MACRO_VAR_Scan(T& t) {
  std::cin >> t;
}
template <typename First, typename... Rest>
void MACRO_VAR_Scan(First& first, Rest&... rest) {
  std::cin >> first;
  MACRO_VAR_Scan(rest...);
}
template <typename T>
void MACRO_VEC_ROW_Init(int n, T& t) {
  t.resize(n);
}
template <typename First, typename... Rest>
void MACRO_VEC_ROW_Init(int n, First& first, Rest&... rest) {
  first.resize(n);
  MACRO_VEC_ROW_Init(n, rest...);
}
template <typename T>
void MACRO_VEC_ROW_Scan(int p, T& t) {
  std::cin >> t[p];
}
template <typename First, typename... Rest>
void MACRO_VEC_ROW_Scan(int p, First& first, Rest&... rest) {
  std::cin >> first[p];
  MACRO_VEC_ROW_Scan(p, rest...);
}
template <class T>
inline T CHMAX(T& a, const T b) {
  return a = (a < b) ? b : a;
}
template <class T>
inline T CHMIN(T& a, const T b) {
  return a = (a > b) ? b : a;
}
template <class T>
using V = std::vector<T>;
template <class T>
using VV = V<V<T>>;
template <typename S, typename T>
std::ostream& operator<<(std::ostream& os, std::pair<S, T> p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<ll, ll>;
using PAIRLL = std::pair<ll, ll>;
constexpr ll INFINT = (1 << 30) - 1;
constexpr ll INFINT_LIM = (1LL << 31) - 1;
constexpr ll INFLL = 1LL << 60;
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);
constexpr double EPS = 1e-10;
constexpr ll MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;
template <class T, size_t N>
void FILL(T (&a)[N], const T& val) {
  for (auto& x : a) x = val;
}
template <class ARY, size_t N, size_t M, class T>
void FILL(ARY (&a)[N][M], const T& val) {
  for (auto& b : a) FILL(b, val);
}
template <class T>
void FILL(std::vector<T>& a, const T& val) {
  for (auto& x : a) x = val;
}
template <class ARY, class T>
void FILL(std::vector<std::vector<ARY>>& a, const T& val) {
  for (auto& b : a) FILL(b, val);
}
struct BIT {
  ll N;
  using TYPE = ll;
  std::vector<TYPE> bit;
  BIT() : N(0) {}
  BIT(ll n) : N(n), bit(N + 1, 0) {}
  BIT(ll n, TYPE init) : N(n), bit(N + 1, init) {}
  void add(ll i, TYPE w) {
    for (ll x = ++i; x <= N; x += x & -x) bit[x] += w;
  }
  TYPE sum(ll i) {
    TYPE res = 0;
    for (ll x = i; x > 0; x -= x & -x) res += bit[x];
    return res;
  }
  TYPE sum(ll i, ll j) { return sum(j) - sum(i); }
  const TYPE& operator[](const ll& i) const { return bit[i]; }
};
signed main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);
  ;
  ll n, k;
  MACRO_VAR_Scan(n, k);
  ;
  std::vector<std::vector<char>> c(n, std::vector<char>(n));
  for (auto& R : c)
    for (auto& w : R) std::cin >> w;
  ;
  BIT baserbit(n);
  ll baserall = 0;
  for (ll i = 0; i < ll(n); ++i) {
    bool ok = true;
    for (ll j = 0; j < ll(n); ++j)
      if (c[i][j] == 'B') ok = false;
    if (ok) baserbit.add(i, 1), ++baserall;
  }
  BIT basecbit(n);
  ll basecall = 0;
  for (ll j = 0; j < ll(n); ++j) {
    bool ok = true;
    for (ll i = 0; i < ll(n); ++i)
      if (c[i][j] == 'B') ok = false;
    if (ok) basecbit.add(j, 1), ++basecall;
  }
  V<BIT> cbit(n - k + 1, BIT(n));
  for (ll i = 0; i < ll(n); ++i) {
    ll sum = 0;
    for (ll j = 0; j < ll(n); ++j) sum += c[i][j] == 'B';
    ll now = 0;
    for (ll j = 0; j < ll(k); ++j) now += c[i][j] == 'B';
    for (ll j = 0; j < ll(n - k + 1); ++j) {
      if (sum - now == 0) cbit[j].add(i, 1);
      if (j + k < n) now += c[i][j + k] == 'B';
      now -= c[i][j] == 'B';
    }
  }
  V<BIT> rbit(n - k + 1, BIT(n));
  for (ll j = 0; j < ll(n); ++j) {
    ll sum = 0;
    for (ll i = 0; i < ll(n); ++i) sum += c[i][j] == 'B';
    ll now = 0;
    for (ll i = 0; i < ll(k); ++i) now += c[i][j] == 'B';
    for (ll i = 0; i < ll(n - k + 1); ++i) {
      if (sum - now == 0) rbit[i].add(j, 1);
      if (i + k < n) now += c[i + k][j] == 'B';
      now -= c[i][j] == 'B';
    }
  }
  ll ans = 0;
  for (ll i = 0; i < ll(n - k + 1); ++i)
    for (ll j = 0; j < ll(n - k + 1); ++j) {
      ll val = cbit[j].sum(i, i + k) + rbit[i].sum(j, j + k);
      val += baserall - baserbit.sum(i, i + k);
      val += basecall - basecbit.sum(j, j + k);
      CHMAX(ans, val);
    }
  std::cout << (ans);
  std::cout << "\n";
  ;
  return 0;
}
