Frog and River

A frog has to cross a river. There are n rocks in the river, using which the frog can leap across the river. On its way across the river the frog can chose to skip a rock, but it cannot skip two consecutive rocks because that would be two far a distance for the frog to hop, also the from would not skip the first rock and the last rock. E.g. if there are 3 rocks, 1,2,3 and 4, there could be three following routes it could take:
1,2,3,4
1,2,3,4
1,3,4
1,2,4 Write a recursive algorithm, that takes a number of rocks' and prints all the feasible paths.