HAM-PATH: given a digraph, is there a path that visits each vertex exactly once.
SHORTEST-SIMPLE-PATH: given a directed graph with edge weights (positive or negative) and two distinguished vertices s and t, find the shortest simple path from s to t. (A simple path is a path that visits each vertex at most once.)
Remark: HAM-PATH is a notoriously difficult problem (NP-complete) for which no poly-time algorithms are known (or likely to exist). This reduction implies that the general version of the shortest path problem (allowing negative edge weights and negative cycles) is intractable.