Ah, you really can’t use it as the power is lower. To add to it the MagSafe connector has a ID within it to ID the charger and the voltage to the system to prevent improper usage.
[http://www.righto.com/2013/06/teardown-and-exploration-of-magsafe.html|Teardown and exploration of Apple's Magsafe connector]
Time to get the correct charger for your system. And please for your safety and your systems use a real Apple charger! The chargers you find on Amazon and eBay are knockoffs which will damage your system and have been known to set buildings ablaze!