server.R

library(shiny)

shinyServer(function(input, output) {
   re <- reactive(function(){
      cat("re called\n",file=stderr())
      input$n
   })

   output$ntext <- reactiveText(function() {
      cat("output$ntext called\n",file=stderr())
      paste(input$n," * 2 =",input$n + re())
   })
})

ui.R

shinyUI(bootstrapPage(
  sliderInput('n', 'N', 1, 10, 1)
))