The problem

A band announces one show in a 50 000 seat stadium. Tickets go on sale Saturday at 10:00. At 9:58, 400 000 fans have the page open and their thumb on the refresh button.

Each fan picks seats on a map, then has 10 minutes to pay. If two fans pay for seat F12 in block 104, one of them stands in the aisle on the night, and support refunds them by hand.

Your job is the part between the click on a seat and the paid ticket.