@startuml
'https://plantuml.com/sequence-diagram

actor User
participant n
participant fact

User -> n: enter n
n -> fact: initialize fact = 1
n -> i: initialize i = 1

group i<=n [if yes]
    fact -> i: fact = fact * i
    i -> i: i = i + 1
else if no
    fact -> User: print(fact)
end

@enduml