ior = io.read
local d = ior("*n", "*l")
local t = {}
for i = 1, 2 do
  local str = ior()
  for j = 1, 7 do
    table.insert(t, str:sub(j, j) == "o")
  end
end
local allmaxlen = 0
for i = 1, 15 - d do
  local maxlen = 0
  local len = 0
  for j = 1, 14 do
    if(t[j] or (i <= j and j < i + d)) then
      len = len + 1
    else
      maxlen = math.max(maxlen, len)
      len = 0
    end
  end
  maxlen = math.max(maxlen, len)
  allmaxlen = math.max(allmaxlen, maxlen)
end
print(allmaxlen)