How to subtract values in fastlane?

In my Fastfile I have method:

lane :calculate_build_version do
    ENV['BUILD_NUMBER'] - BuildNumberFactory.make
  end

BuldNumberFactory code is:

class BuildNumberFactory
  class << self
    def make
      `git rev-list HEAD --count`
    end
  end
end

During build I get error:

Fastfile:22:in `block (2 levels) in parsing_binding': [!] undefined method `-' for "192":String (NoMethodError)

How can I fix this and perform subtract?