Changes

If you are on a low budget: The security at the entry of the metro stations is watching for people who enter, that they validate their tickets. Even if they are there you can ask them friendly if they let you enter without ticket as you don't have much money. Worked with me 3 of 4 times, basically outside the very centre of the city. In the centre they are more strict and you need to be careful when you change the lines there.