NEW YORK CITY (CNN NEWSOURCE) — Japanese inventors have found a way to discourage rush-hour groping in crowded areas.

This stamp allows victims of harassment to mark their assailants with invisible ink. The mark is in the shape of a hand and shows up under certain types of light.

The company, Shachihata Inc., says it developed the stamp after seeing discussions on social media. Women in Japan have long looked for ways to stop sexual harassment on crowded trains and public transportation known as “chikan.”

Shachihata hopes the stamp will act as a deterrent to would-be attackers.

A 500-stamp test-run of the prototype sold out in less than 30 minutes online. It is currently going for 2,500 yen, or about $24.