Lillian Barrios-Paoli -- a former nun who has worked for Ed Koch, Rudy Giuliani and Mike Bloomberg -- will soon join the de Blasio administration as deputy mayor for health and human services.

Smart choice, but she'll need to make miracles happen.

Her first challenge is devising a way to move thousands of homeless people from city shelters into permanent housing. The shelter census today stands at more than 51,000 -- about 22,000 children and 29,000 adults.

That's a distressingly high level not seen since the 1930s -- a crisis the city must tackle swiftly.

But any workable strategy will be costly -- at a time when money is tight at all levels of government.

Unfortunately, Mayor-elect Bill de Blasio sounds as if he's still stuck in campaign mode. At a news conference last week to introduce his new deputy mayor, he basically blamed the Bloomberg administration for cascading homelessness in the five boroughs.

De Blasio said Bloomberg backed away from a homeless strategy that worked and "led us down a bad path."

Not quite. In 2007 Bloomberg unveiled the Advantage program, a signature rent-subsidy plan aimed at homeless families and individuals. Funded by the city and state, it succeeded at first. Within a year, 19,000 individuals and families had signed up.

But in 2011 the bottom fell out of the program when the state hit a budget crunch. It dropped its support, and the city couldn't afford to go it alone.

Advantage ultimately was a victim of the Great Recession and its shrinking wages, rising unemployment and slashed public services.

So where does that leave Barrios-Paoli?

The city will pay out an estimated $5 billion this year alone for shelter and related social services. If de Blasio wants to spring for more, where will it come from?

De Blasio will take office with 152 city labor contracts to settle. Albany isn't likely to let him increase taxes, and the man he loves to blame for the city's problems will be gone.

You'll need to work some magic, Ms. Barrios-Paoli.