2 glass balls,100-story building

You are holding two glass balls in a 100-story building. If a ball is dropped off the side of the building from floor N or any floor below N, it will not break. If it's dropped from any floor above N, it will break. What is the fewest number of drops, k, you need to guarantee that you can find N?

Note: you want a strategy that will minimize the number of drops needed in the worst case scenario.

Source: The green book