A group of N stations share a 56-kbps pure ALOHA channels. Each station outputs a 1000-bit frame

Question:

A group of N stations share a 56-kbps pure ALOHA channels. Each station outputs a 1000-bit frame on an average of once every 100 sec, even if the previous one has not yet been sent (e.g., the stations can buffer outgoing frames). What is the maximum value of N?