checkboxGroupInput - set minimum and maximum number of selections - ticks

Here is example code with check-box group input:

library(shiny)

server <- function(input, output) {
  output$Selected <- renderText({
    paste(input$SelecetedVars,collapse=",")
  })
}

ui <- fluidPage(
  sidebarLayout(
    sidebarPanel(
      checkboxGroupInput("SelecetedVars", "MyList:",
                         paste0("a",1:5), selected = "a1")
    ),
    mainPanel(textOutput("Selected"))
  )
)

shinyApp(ui = ui, server = server)

As you you can see from image above we can select as many as we want, in this case 4 out of 5.

How can I set minimum and maximum number of ticks? I need minimum of 1 option checked and maximum of 3 options checked. i.e.: prevent unticking the last tick, and prevent ticking when 3 options were already ticked.