How to add css link to .jade file

I am creating a nodejs application and i use the jade engine view

I am using bootstrap so in jade i import it in the following way:

html
    head
        title=title
        link(href('//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css', rel='stylesheet'))
    body
        div.container
            div.jumbotron
                h1 Hello!

                if user
                    p Welcome, #{user.fullName}
                    p
                        a.small(href="profile") Edit my profile
                    a.btn.btn-primary(href="/logout") Logout
                else
                    p Welcome to my app, ready to get started?
                    p
                        a.btn.btn-primary(href="/login") Login now
                    p
                        span.small Don't have an account=
                        span  
                        a.small(href="/register") Register now

Although when i run this. I get the following error:

SyntaxError: C:\Users\user\Documents\SIGMA\NodeDotJS\real_examples\login-page\views\home.jade:4
   2| head 
   3| title=title 
 > 4| link(href('//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css', rel='stylesheet'))

Unexpected token )
   at Function (<anonymous>)
   at assertExpression (C:\Users\user\Documents\SIGMA\NodeDotJS\real_examples\login-page\node_modules\jade\lib\lexer.js:30:3)
   at Object.Lexer.attrs (C:\Users\user\Documents\SIGMA\NodeDotJS\real_examples\login-page\node_modules\jade\lib\lexer.js:661:20)
   at Object.Lexer.next (C:\Users\user\Documents\SIGMA\NodeDotJS\real_examples\login-page\node_modules\jade\lib\lexer.js:922:15)
   at Object.Lexer.lookahead (C:\Users\user\Documents\SIGMA\NodeDotJS\real_examples\login-page\node_modules\jade\lib\lexer.js:113:46)
   at Parser.lookahead (C:\Users\user\Documents\SIGMA\NodeDotJS\real_examples\login-page\node_modules\jade\lib\parser.js:102:23)
   at Parser.peek (C:\Users\user\Documents\SIGMA\NodeDotJS\real_examples\login-page\node_modules\jade\lib\parser.js:79:17)
   at Parser.tag (C:\Users\user\Documents\SIGMA\NodeDotJS\real_examples\login-page\node_modules\jade\lib\parser.js:752:22)
   at Parser.parseTag (C:\Users\user\Documents\SIGMA\NodeDotJS\real_examples\login-page\node_modules\jade\lib\parser.js:738:17)
   at Parser.parseExpr (C:\Users\user\Documents\SIGMA\NodeDotJS\real_examples\login-page\node_modules\jade\lib\parser.js:211:21)

What is wrong with this jade code? i open ( and i close it.