Since we have the upper limit 10¹⁵ for all of our numbers , we can first convert them to decimal (and store the answer in a 64-bit integer) and then to the target base.

For converting a number to decimal we first set the decimal variable to 0, then at each step we multiply it by the base and add the left-most digit's equivalent in base 10.

We had some tricky test cases for this one which got many people :

test #51:

2 10

0

many codes printed nothing for this one, this was also the most used hack for this problem

test #54:

12 2

000..00A

a sample of having initial zeros in input

test #55:

17 17

0000000...000

There were many people who just printed the input if bases were equal

there were two nice extremal hacks:

2 R

101110111000 

and

10 2

1000000000000000