A lily flower has began growing on a lake. In the first day it has 0.5 square meters. Every day it grows twice as big as the last day( so the second day it's 1 meter square , third day 2 meters square etc.).
In 37 day it grows so big that it covers the entire surface of the lake.
Question: How long does it take for the lily flower to grow so that it covers half of the lake surface?