Shortest Impossible Sequence of Rolls

You're provided with an integer array, rolls, of length n, and an integer, k. You roll a k-sided dice numbered from 1 to k, n times, where the result of the ith roll is rolls[i].

Your task is to print the length of the shortest sequence of rolls that cannot be obtained from rolls.

A sequence of rolls of length len is defined as the result of rolling a k-sided dice len times.