Chipotle Mexican Grill unveiled an online game called “Guac Hunter” that gives players an opportunity to win free chip and guacamole, the latest charm offensive to win back diners that have spurned the Mexican restaurant chain in recent months.

The company’s ongoing charm offensive is now being delivered in the form of a digital photo hunt where users are shown a series of two images that look similar, but must spot the differences before time runs out. Players are rewarded a free order of the chips and guacamole side (whether or not they win).

Notably, there are a few restrictions for this prize. Users must play “Guac Hunter” between March 21 to March 31 to be eligible for the prize. And all offers expire by April 10.

The initiative is the latest by Chipotle cmg to win back business that it has lost since food safety concerns first started to surface in October. In the wake of an E. Coli outbreak last year, Chipotle’s sales and stock price have been badly dented. Sales at established restaurants dropped 26.1% in February, following a steep 36.4% drop in January. Though declines are moderating, Chipotle is projecting a loss of $1 per share this quarter.

To help win back that lost business, Chipotle has had to spend more money on advertising and also on giveaways. As Fortune reported earlier this month, the chain has opted to extend a promotion to give away free burritos. Executives are hopeful those efforts can convince people that the food sold at Chipotle is safe.

Mark Crumpacker, chief creative and development officer at Chipotle, alluded to that strategy in his statement when announcing the launch of “Guac Hunter.”

“This game allows our customers to enjoy some chips and guac on us, and perhaps encourage others to give our guac a try and see what they’ve been missing,” he said.