Update updateselectInput() based on values of another updateSelectInput()

In the app below I want to update the second updateSelectInput() I have in order to exclude the values selected in the first updateSelectInput() after uploading a file. Normally the file uploads the dataset used for shiny widgets' values.

## app.R ##library(shiny)library(shinydashboard)ui <- dashboardPage(  dashboardHeader(),  dashboardSidebar(    fileInput("file1", "Choose CSV File", accept = ".csv"),    selectInput("col","Pick a column to change its name",                choices = colnames(iris)[-c(1)]),    selectInput("col2","Pick a column to change its name",                choices = colnames(iris)[-c(1)])  ),  dashboardBody(    ))server <- function(session,input, output) {  observeEvent(input$file1, {    updateSelectInput(session, "col", label = "Select target country", choices = colnames(iris)[-c(1)])    ex_names <- colnames(iris)[-1]    ex_names <- ex_names[! ex_names %in% input$col]    updateSelectInput(session, "col2", label = "Select reference countries", choices =colnames(iris)[-c(1)] )    })}shinyApp(ui, server)