Hey /sci/, brainlet here. I have a question, suppose I have n numbers and n + 1 letters, I want to list all possible sequences with this rule: whenever the number of numbers and letters are the same, I have to add a number to the sequence. For example, for (1, 2) and (a, b, c) 12abc, 1a2bc, 1ab2c, a12bc, a1b2c... are okay, but 1abc2 or ab12c not. How do I calculate the number of possible sequences might be generated?