#include <bits/stdc++.h>
using namespace std;
const int dx[4] = {0, 1, -1, 0}, dy[4] = {-1, 0, 0, 1};
int n, m;
char g[60][60];
bool col[60][60][2][4];
bool tmp[60][60][2][4];
bool inrange(int x, int y) { return x >= 0 && x < n && y >= 0 && y < m; }
void solve(int x, int y, int turn, int dir) {
  int nx = x + dx[dir], ny = y + dy[dir];
  if (inrange(nx, ny) && g[nx][ny] == 'B' && tmp[nx][ny][turn][dir] == false) {
    tmp[nx][ny][turn][dir] = tmp[nx][ny][1][dir] = true;
    solve(nx, ny, turn, dir);
  }
  if (turn == 1) return;
  for (int i = 0; i < 4; i++) {
    if (i == dir) continue;
    nx = x + dx[i], ny = y + dy[i];
    if (inrange(nx, ny) && g[nx][ny] == 'B' && tmp[nx][ny][1][i] == false) {
      tmp[nx][ny][1][i] = true;
      solve(nx, ny, 1, i);
    }
  }
  return;
}
bool check() {
  for (int i = 0; i < n; i++)
    for (int j = 0; j < m; j++)
      if (g[i][j] == 'B') {
        bool ok = false;
        for (int x = 0; x < 2 && !ok; x++)
          for (int y = 0; y < 4 && !ok; y++)
            if (tmp[i][j][x][y]) ok = true;
        if (!ok) return false;
      }
  return true;
}
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 0; i < n; i++)
    for (int j = 0; j < m; j++) {
      cin >> g[i][j];
    }
  for (int i = 0; i < n; i++)
    for (int j = 0; j < m; j++)
      if (g[i][j] == 'B') {
        memset(tmp, false, sizeof(tmp));
        for (int k = 0; k < 4; k++)
          tmp[i][j][0][k] = tmp[i][j][1][k] = true, solve(i, j, 0, k);
        if (!check()) {
          cout << "NO" << endl;
          return 0;
        }
      }
  cout << "YES" << endl;
  return 0;
}
