  A $2\otimes 2$ unitary operation is called a perfect entangler if it can
generate a maximally entangled state from some unentangled input. We study the
following question: How many runs of a given two-qubit entangling unitary
operation is required to simulate some perfect entangler with one-qubit unitary
operations as free resources? We completely solve this problem by presenting an
analytical formula for the optimal number of runs of the entangling operation.
Our result reveals an entanglement strength of two-qubit unitary operations.
