Buggy python submission (verdict: Time Limit Exceeded):
def calcb(lx,ly,rx,ry):
  if rx<0 or ry<0 or lx>rx or ly>ry:
    return 0
  ret=acum1[ry][rx]
  if lx!=0:
    ret-=acum1[ry][lx-1]
  if ly!=0:
    ret-=acum1[ly-1][rx]
  if lx!=0 and ly!=0:
    ret+=acum1[ly-1][lx-1]
  return ret

def calcw(lx,ly,rx,ry):
  if rx<0 or ry<0 or lx>rx or ly>ry:
    return 0
  ret=acum2[ry][rx]
  if lx!=0:
    ret-=acum2[ry][lx-1]
  if ly!=0:
    ret-=acum2[ly-1][rx]
  if lx!=0 and ly!=0:
    ret+=acum2[ly-1][lx-1]
  return ret

n,k=map(int,input().split())
board1=[[0]*(2*k) for _ in range(2*k)]
board2=[[0]*(2*k) for _ in range(2*k)]
acum1=[[0]*(2*k) for _ in range(2*k)]
acum2=[[0]*(2*k) for _ in range(2*k)]
for _ in range(n):
  x,y,c=input().split()
  x,y=int(x)%(2*k),int(y)%(2*k)
  if c=='B':
    board1[y][x]+=1
  if c=='W':
    board2[y][x]+=1
for i in range(2*k):
  for j in range(2*k):
    acum1[i][j]=board1[i][j]
    acum2[i][j]=board2[i][j]
    if i!=0:
      acum1[i][j]+=acum1[i-1][j]
      acum2[i][j]+=acum2[i-1][j]
    if j!=0:
      acum1[i][j]+=acum1[i][j-1]
      acum2[i][j]+=acum2[i][j-1]
    if i!=0 and j!=0:
      acum1[i][j]-=acum1[i-1][j-1]
      acum2[i][j]-=acum2[i-1][j-1]
ans=0
for i in range(k):
  for j in range(k):
    tmp1=0
    tmp2=0
    lx,ly=i,j
    rx,ry=lx+k-1,ly+k-1
    #1個目のマス
    tmp1+=calcb(lx,ly,rx,ry)
    tmp2+=calcw(lx,ly,rx,ry)
    #左上のマス
    tlx,tly=0,0
    trx,ttry=lx-1,ly-1
    tmp1+=calcb(tlx,tly,trx,ttry)
    tmp2+=calcw(tlx,tly,trx,ttry)
    #右上のマス
    tlx,tly=rx+1,0
    trx,ttry=2*k-1,ly-1
    tmp1+=calcb(tlx,tly,trx,ttry)
    tmp2+=calcw(tlx,tly,trx,ttry)
    #左下のマス
    tlx,tly=0,ry+1
    trx,ttry=lx-1,2*k-1
    tmp1+=calcb(tlx,tly,trx,ttry)
    tmp2+=calcw(tlx,tly,trx,ttry)
    #右下のマス
    tlx,tly=rx+1,ry+1
    trx,ttry=2*k-1,2*k-1
    tmp1+=calcb(tlx,tly,trx,ttry)
    tmp2+=calcw(tlx,tly,trx,ttry)
    #上のマス
    tlx,tly=lx,0
    trx,ttry=rx,ly-1
    tmp1+=calcw(tlx,tly,trx,ttry)
    tmp2+=calcb(tlx,tly,trx,ttry)
    #左のマス
    tlx,tly=0,ly
    trx,ttry=lx-1,ry
    tmp1+=calcw(tlx,tly,trx,ttry)
    tmp2+=calcb(tlx,tly,trx,ttry)
    #右のマス
    tlx,tly=rx+1,ly
    trx,ttry=2*k-1,ry
    tmp1+=calcw(tlx,tly,trx,ttry)
    tmp2+=calcb(tlx,tly,trx,ttry)
    #下のマス
    tlx,tly=lx,ry+1
    trx,ttry=rx,2*k-1
    tmp1+=calcw(tlx,tly,trx,ttry)
    tmp2+=calcb(tlx,tly,trx,ttry)
    ans=max(ans,tmp1,tmp2)
print(ans)

Fixed submission:
def calcb(lx,ly,rx,ry):
  if rx<0 or ry<0 or lx>rx or ly>ry:
    return 0
  ret=acum1[ry*2*k+rx]
  if lx!=0:
    ret-=acum1[ry*2*k+lx-1]
  if ly!=0:
    ret-=acum1[(ly-1)*2*k+rx]
  if lx!=0 and ly!=0:
    ret+=acum1[(ly-1)*2*k+lx-1]
  return ret

def calcw(lx,ly,rx,ry):
  if rx<0 or ry<0 or lx>rx or ly>ry:
    return 0
  ret=acum2[ry*2*k+rx]
  if lx!=0:
    ret-=acum2[ry*2*k+lx-1]
  if ly!=0:
    ret-=acum2[(ly-1)*2*k+rx]
  if lx!=0 and ly!=0:
    ret+=acum2[(ly-1)*2*k+lx-1]
  return ret

n,k=map(int,input().split())
board1=[0]*(4*k*k)
board2=[0]*(4*k*k)
acum1=[0]*(4*k*k)
acum2=[0]*(4*k*k)
for _ in range(n):
  x,y,c=input().split()
  x,y=int(x)%(2*k),int(y)%(2*k)
  if c=='B':
    board1[y*2*k+x]+=1
  if c=='W':
    board2[y*2*k+x]+=1
for i in range(2*k):
  for j in range(2*k):
    acum1[i*2*k+j]=board1[i*2*k+j]
    acum2[i*2*k+j]=board2[i*2*k+j]
    if i!=0:
      acum1[i*2*k+j]+=acum1[(i-1)*2*k+j]
      acum2[i*2*k+j]+=acum2[(i-1)*2*k+j]
    if j!=0:
      acum1[i*2*k+j]+=acum1[i*2*k+j-1]
      acum2[i*2*k+j]+=acum2[i*2*k+j-1]
    if i!=0 and j!=0:
      acum1[i*2*k+j]-=acum1[(i-1)*2*k+j-1]
      acum2[i*2*k+j]-=acum2[(i-1)*2*k+j-1]
ans=0
for i in range(k):
  for j in range(k):
    tmp1=0
    tmp2=0
    lx,ly=i,j
    rx,ry=lx+k-1,ly+k-1
    #1個目のマス
    tmp1+=calcb(lx,ly,rx,ry)
    tmp2+=calcw(lx,ly,rx,ry)
    #左上のマス
    tlx,tly=0,0
    trx,ttry=lx-1,ly-1
    tmp1+=calcb(tlx,tly,trx,ttry)
    tmp2+=calcw(tlx,tly,trx,ttry)
    #右上のマス
    tlx,tly=rx+1,0
    trx,ttry=2*k-1,ly-1
    tmp1+=calcb(tlx,tly,trx,ttry)
    tmp2+=calcw(tlx,tly,trx,ttry)
    #左下のマス
    tlx,tly=0,ry+1
    trx,ttry=lx-1,2*k-1
    tmp1+=calcb(tlx,tly,trx,ttry)
    tmp2+=calcw(tlx,tly,trx,ttry)
    #右下のマス
    tlx,tly=rx+1,ry+1
    trx,ttry=2*k-1,2*k-1
    tmp1+=calcb(tlx,tly,trx,ttry)
    tmp2+=calcw(tlx,tly,trx,ttry)
    #上のマス
    tlx,tly=lx,0
    trx,ttry=rx,ly-1
    tmp1+=calcw(tlx,tly,trx,ttry)
    tmp2+=calcb(tlx,tly,trx,ttry)
    #左のマス
    tlx,tly=0,ly
    trx,ttry=lx-1,ry
    tmp1+=calcw(tlx,tly,trx,ttry)
    tmp2+=calcb(tlx,tly,trx,ttry)
    #右のマス
    tlx,tly=rx+1,ly
    trx,ttry=2*k-1,ry
    tmp1+=calcw(tlx,tly,trx,ttry)
    tmp2+=calcb(tlx,tly,trx,ttry)
    #下のマス
    tlx,tly=lx,ry+1
    trx,ttry=rx,2*k-1
    tmp1+=calcw(tlx,tly,trx,ttry)
    tmp2+=calcb(tlx,tly,trx,ttry)
    ans=max(ans,tmp1,tmp2)
print(ans)

Buggy python submission (verdict: Runtime Error):
def calcb(lx,ly,rx,ry):
  if rx<0 or ry<0 or lx>rx or ly>ry:
    return 0
  ret=acum1[ry][rx]
  if lx!=0:
    ret-=acum1[ry][lx-1]
  if ly!=0:
    ret-=acum1[ly-1][rx]
  if lx!=0 and ly!=0:
    ret+=acum1[ly-1][lx-1]
  return ret

def calcw(lx,ly,rx,ry):
  if rx<0 or ry<0 or lx>rx or ly>ry:
    return 0
  ret=acum2[ry][rx]
  if lx!=0:
    ret-=acum2[ry][lx-1]
  if ly!=0:
    ret-=acum2[ly-1][rx]
  if lx!=0 and ly!=0:
    ret+=acum2[ly-1][lx-1]
  return ret

n,k=map(int,input().split())
board1=[[0]*(2*k) for _ in range(2*k)]
board2=[[0]*(2*k) for _ in range(2*k)]
acum1=[[0]*(2*k) for _ in range(2*k)]
acum2=[[0]*(2*k) for _ in range(2*k)]
for _ in range(n):
  x,y,c=input().split()
  x,y=int(x)%(2*k),int(y)%(2+k)
  if c=='B':
    board1[y][x]+=1
  if c=='W':
    board2[y][x]+=1
for i in range(2*k):
  for j in range(2*k):
    acum1[i][j]=board1[i][j]
    acum2[i][j]=board2[i][j]
    if i!=0:
      acum1[i][j]+=acum1[i-1][j]
      acum2[i][j]+=acum2[i-1][j]
    if j!=0:
      acum1[i][j]+=acum1[i][j-1]
      acum2[i][j]+=acum2[i][j-1]
    if i!=0 and j!=0:
      acum1[i][j]-=acum1[i-1][j-1]
      acum2[i][j]-=acum2[i-1][j-1]
ans=0
for i in range(k):
  for j in range(k):
    tmp=0
    lx,ly=i,j
    rx,ry=lx+k-1,ly+k-1
    #1個目の黒マス
    tmp+=calcb(lx,ly,rx,ry)
    #左上の黒マス
    tlx,tly=0,0
    trx,ttry=lx-1,ly-1
    tmp+=calcb(tlx,tly,trx,ttry)
    #右上の黒マス
    tlx,tly=rx+1,0
    trx,ttry=2*k-1,ly-1
    tmp+=calcb(tlx,tly,trx,ttry)
    #左下の黒マス
    tlx,tly=0,ry+1
    trx,ttry=lx-1,2*k-1
    tmp+=calcb(tlx,tly,trx,ttry)
    #右下の黒マス
    tlx,tly=rx+1,ry+1
    trx,ttry=2*k-1,2*k-1
    tmp+=calcb(tlx,tly,trx,ttry)
    #上の白マス
    tlx,tly=lx,0
    trx,ttry=rx,ly-1
    tmp+=calcw(tlx,tly,trx,ttry)
    #左の白マス
    tlx,tly=0,ly
    trx,ttry=lx-1,ry
    tmp+=calcw(tlx,tly,trx,ttry)
    #右の白マス
    tlx,tly=rx+1,ly
    trx,ttry=2*k-1,ry
    tmp+=calcw(tlx,tly,trx,ttry)
    #下の白マス
    tlx,tly=lx,ry+1
    trx,ttry=rx,2*k-1
    tmp+=calcw(tlx,tly,trx,ttry)
    ans=max(ans,tmp)
print(ans)

Fixed submission:
def calcb(lx,ly,rx,ry):
  if rx<0 or ry<0 or lx>rx or ly>ry:
    return 0
  ret=acum1[ry*2*k+rx]
  if lx!=0:
    ret-=acum1[ry*2*k+lx-1]
  if ly!=0:
    ret-=acum1[(ly-1)*2*k+rx]
  if lx!=0 and ly!=0:
    ret+=acum1[(ly-1)*2*k+lx-1]
  return ret

def calcw(lx,ly,rx,ry):
  if rx<0 or ry<0 or lx>rx or ly>ry:
    return 0
  ret=acum2[ry*2*k+rx]
  if lx!=0:
    ret-=acum2[ry*2*k+lx-1]
  if ly!=0:
    ret-=acum2[(ly-1)*2*k+rx]
  if lx!=0 and ly!=0:
    ret+=acum2[(ly-1)*2*k+lx-1]
  return ret

n,k=map(int,input().split())
board1=[0]*(4*k*k)
board2=[0]*(4*k*k)
acum1=[0]*(4*k*k)
acum2=[0]*(4*k*k)
for _ in range(n):
  x,y,c=input().split()
  x,y=int(x)%(2*k),int(y)%(2*k)
  if c=='B':
    board1[y*2*k+x]+=1
  if c=='W':
    board2[y*2*k+x]+=1
for i in range(2*k):
  for j in range(2*k):
    acum1[i*2*k+j]=board1[i*2*k+j]
    acum2[i*2*k+j]=board2[i*2*k+j]
    if i!=0:
      acum1[i*2*k+j]+=acum1[(i-1)*2*k+j]
      acum2[i*2*k+j]+=acum2[(i-1)*2*k+j]
    if j!=0:
      acum1[i*2*k+j]+=acum1[i*2*k+j-1]
      acum2[i*2*k+j]+=acum2[i*2*k+j-1]
    if i!=0 and j!=0:
      acum1[i*2*k+j]-=acum1[(i-1)*2*k+j-1]
      acum2[i*2*k+j]-=acum2[(i-1)*2*k+j-1]
ans=0
for i in range(k):
  for j in range(k):
    tmp1=0
    tmp2=0
    lx,ly=i,j
    rx,ry=lx+k-1,ly+k-1
    #1個目のマス
    tmp1+=calcb(lx,ly,rx,ry)
    tmp2+=calcw(lx,ly,rx,ry)
    #左上のマス
    tlx,tly=0,0
    trx,ttry=lx-1,ly-1
    tmp1+=calcb(tlx,tly,trx,ttry)
    tmp2+=calcw(tlx,tly,trx,ttry)
    #右上のマス
    tlx,tly=rx+1,0
    trx,ttry=2*k-1,ly-1
    tmp1+=calcb(tlx,tly,trx,ttry)
    tmp2+=calcw(tlx,tly,trx,ttry)
    #左下のマス
    tlx,tly=0,ry+1
    trx,ttry=lx-1,2*k-1
    tmp1+=calcb(tlx,tly,trx,ttry)
    tmp2+=calcw(tlx,tly,trx,ttry)
    #右下のマス
    tlx,tly=rx+1,ry+1
    trx,ttry=2*k-1,2*k-1
    tmp1+=calcb(tlx,tly,trx,ttry)
    tmp2+=calcw(tlx,tly,trx,ttry)
    #上のマス
    tlx,tly=lx,0
    trx,ttry=rx,ly-1
    tmp1+=calcw(tlx,tly,trx,ttry)
    tmp2+=calcb(tlx,tly,trx,ttry)
    #左のマス
    tlx,tly=0,ly
    trx,ttry=lx-1,ry
    tmp1+=calcw(tlx,tly,trx,ttry)
    tmp2+=calcb(tlx,tly,trx,ttry)
    #右のマス
    tlx,tly=rx+1,ly
    trx,ttry=2*k-1,ry
    tmp1+=calcw(tlx,tly,trx,ttry)
    tmp2+=calcb(tlx,tly,trx,ttry)
    #下のマス
    tlx,tly=lx,ry+1
    trx,ttry=rx,2*k-1
    tmp1+=calcw(tlx,tly,trx,ttry)
    tmp2+=calcb(tlx,tly,trx,ttry)
    ans=max(ans,tmp1,tmp2)
print(ans)