n, m = gets.split.map(&:to_i)

if n == m
  puts 1
  exit
end
if n == 3 && m < 3
  puts -1
  exit
end

if n % 2 == 1 && m == n / 2 + 1
  puts (n / m.to_f).ceil + 1
  exit
end

puts (n / m.to_f).ceil