local function getprimes(x)
  local primes = {}
  local allnums = {}
  for i = 1, x do allnums[i] = true end
  for i = 2, x do
    if(allnums[i]) then
      table.insert(primes, i)
      local lim = math.floor(x / i)
      for j = 2, lim do
        allnums[j * i] = false
      end
    end
  end
  return primes
end

local n, p = io.read("*n", "*n")
local primes = getprimes(n)
if(p == 1) then
  print(1)
elseif(p * 2 <= n) then
  local ret = n - 1
  local lim = math.floor(n / 2)
  for i = 1, #primes do
    if(primes[i] ~= p and lim < primes[i]) then
      ret = ret - 1
      break
    end
  end
  print(ret)
else
  local isp = false
  for i = 1, #primes do
    if(primes[i] == p) then
      isp = true
      print(1)
      break
    end
  end
  if(not isp) then
    local ret = n - 1
    local lim = math.floor(n / 2)
    for i = 1, #primes do
      if(primes[i] ~= p and lim < primes[i]) then
        ret = ret - 1
        break
      end
    end
    print(ret)
  end
end