TypeError: express is not a function

Hi I'm trying to use jade and express together. But it is giving me TypeError. I made npm init, npm install express -save and then node app.js. But it is giving me same error "TypeError: express is not a function"

// Module dependencies
var express = require('express')
  , nib = require('nib')
  , mysql = require('mysql')

 var app = express();

var app = module.exports = app.createServer();


app.set('views', __dirname + '/views')
app.set('view engine', 'jade')
app.use(express.logger('dev'))

And package.json :

{
  "name": "reddit-node-mysql",
  "description": "A demo of how to use Express and MySQL together",
  "author": "Clarence Leung <github@clarle>",
  "version": "0.0.1",
  "private": true,
  "dependencies": {
    "express": "^2.5.11",
    "mysql": "~2.0",
    "nib": "^1.1.2",
    "jade": "^1.0.4"
  },
  "main": "app.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "license": "ISC",
  "devDependencies": {}
}

Solution My modules' version outdated so they can't installed well. I updated version of them, now working.