class Numeric
    def inches
        self
    end
    
    def feet
        self * 12.inches
    end

    def yards
        self * 3.feet
    end

    def miles
        self * 5280.feet
    end

    def back
        self * -1
    end

    def forward
        self
    end

end

puts 10.miles.back
puts 2.feet.forward