EDIT: This puzzle originates from the "International Mathematics Tournament of the Towns", and was published in the book "S.M.A.R.T circle overview" by Professor Andy Liu. The author has (albeit a little retroactively) granted us the permission to use the puzzle.
Ten thieves, ranked A to J, are trying to cross a river in a boat requiring two rowers. Unfortunately, if the ranks of any two in the boat differ by more than 1, those two will refuse to stay in the boat. This constraint means they can’t get across the river. Their leader, with a rank of A, asks Ali Baba for help and Ali Baba replies, “If you give me a rank of A, equal to yours, we can all cross the river.” The leader agrees.
How many one-way crossings are the least required to get Ali Baba and the 10 thieves across the river?
More than 2 people can accumulate in the boat. Boat is operated by 2 rowers. So a single person cannot row it on his own.