Starting with two positive integers a and b, consider the sequence in which the next number is the digit in the ones place of the sum of the previous two numbers. For example, if a=1 and b=1, the sequence is

1, 1, 2, 3, 5, 8, 3, 1, 4, 5, 9, 4, 3, 7, 0, ...

Write a function in python

mystery(a, b)

that returns the length of the sequence when the last two numbers repeat the values of a and b for the first time. (When a=1 and b=1, the function should return 62.)