A frog wants to cross a river that is 11 feet across.
There are 10 stones in a line leading across the river, separated by 1 foot. He can either jump to the next stone or jump over a stone, but always moving forward (toward the other side of the river).
How many different ways can he cross the river? Remember, just getting to the 10th rock won't be quite enough, he would need to make one final jump to get across.
For example, one way to cross would be (where 1 means he goes to the next rock and 2 means he skips one):