The mayoral election has a transferable voting system, so you can vote for your favourite candidate first and then your second choice. In this system assuming one candidate does not get 50%, all but the first two candidates are eliminated and their votes redistributed.

So, in the case of SophieBee she could vote Paddick 1, Livingstone 2. If Paddick beats Boris in the first round then your vote does not transfer, but if the run off is between Boris and Ken, then your vote will go to Boris.

However, voting Green 1, UKIP 2, will probably be something of a wasted vote as both are likely to be eliminated in the first round and your transfer vote will be worthless. So make sure you predict the top two candidates correctly if you think your first choice may not get through!