I started using Emacs and I wrote a text in Greek. The problem is that the stress that is used upon some letters (ό, ύ, ί, ύ) doesn't work! 

It is usually inserted by pressing the `;` (I mean by pressing the key next to L) and then the letter upon which will be inserted. That's standard for a Greek keyboard. When I press the `;` it says:

    <dead-acute> is undefined. 

`C-h c ;` says the same thing.

What can I do to be able to use this key?

It works in other applications, such as a browser. I use Ubuntu 14.04 and I installed its `emacs` package (Emacs 24.3.1).