local mfl = math.floor
local mce, msq = math.ceil, math.sqrt
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 = mfl(x / i)
      for j = 2, lim do
        allnums[j * i] = false
      end
    end
  end
  return primes
end

local function getdivisorparts(x, primes)
  local prime_num = #primes
  local tmp = {}
  local lim = mce(msq(x))
  local primepos = 1
  local dv = primes[primepos]
  while primepos <= prime_num and dv <= lim do
    if x % dv == 0 then
      local asdf = {}
      asdf.p = dv
      asdf.cnt = 1
      x = x / dv
      while x % dv == 0 do
        x = x / dv
        asdf.cnt = asdf.cnt + 1
      end
      table.insert(tmp, asdf)
      lim = mce(msq(x))
    end
    if primepos == prime_num then break end
    primepos = primepos + 1
    dv = primes[primepos]
  end
  if x ~= 1 then
    local asdf = {}
    asdf.p, asdf.cnt = x, 1
    table.insert(tmp, asdf)
  end
  return tmp
end

local function getdivisor(divisorparts)
  local t = {}
  local pat = 1
  local len = #divisorparts
  local allpat = 1
  for i = 1, len do
    allpat = allpat * (1 + divisorparts[i].cnt)
  end
  for t_i_pat = 0, allpat - 1 do
    local div = allpat
    local i_pat = t_i_pat
    local ret = 1
    for i = 1, len do
      div = mfl(div / (divisorparts[i].cnt + 1))
      local mul = mfl(i_pat / div)
      i_pat = i_pat % div
      for j = 1, mul do
        ret = ret * divisorparts[i].p
      end
    end
    table.insert(t, ret)
  end
  table.sort(t)
  return t
end


local n, s = io.read("*n", "*n")
local lim = math.ceil(math.sqrt(n))

local function solve(x)
  local tn = n
  local c = 0
  while 0 < tn do
    c = c + tn % x
    tn = mfl(tn / x)
  end
  return c == s
end

local found = false
for x = 2, lim - 1 do
  if solve(x) then
    found = true
    print(x)
    break
  end
end
if not found then
  if n == s then
    print(n + 1)
  elseif s < n then
    local mul = n - s
    local primes = getprimes(lim)
    local divp = getdivisorparts(mul, primes)
    local divs = getdivisor(divp)
    for i = 2, #divs do
      local cand = divs[i]
      local a1 = mfl(mul / cand)
      if a1 <= cand then
        local a0 = s - a1
        if 0 <= a0 and a0 <= cand then
          found = true
          print(cand + 1)
          break
        end
      end
    end
  end
  if not found then
    print(-1)
  end
end
