How to reduce the hamiltonian path problem to 1/2 hamiltonian path problem Task:
A Hamiltonian path of a graph is a path that visits all nodes of the graph exactly once. The hamiltion path problem (HPP) consists in deciding whether a given graph has such a path. Similarly, the 1/2 hamiltion path problem (1/2-HPP) asks for a path that visits exactly half of the nodes. 
Show that: $HPP \le_{P} 1/2-HPP$

Question: Can anyone give me a hint on how to prove $HPP \le_{P} 1/2-HPP$
Double the graph size: make two clones of the input $G_1,G_2$ and now create the (not connected) graph $\hat G$ that will consist of the two clones $G_1,G_2$. Now a half-hamiltonian path in $\hat G$ is either going through all $G_1$ or all $G_2$ (but not both, since they are not connected) and thus would be a hamiltonian path in $G$.