Hi, thanks for asking!

This problem (584A) tests your creativity skills, and you may have to learn how to construct the corresponding number.

One solution is to just construct a string of "t"s (when t is not 10) with n digits, and clearly the resulting number is a multiple of t. Here is my code:

n,t=input().split()
n=int(n)
t=int(t)
if t==10:
    if n==1:print(-1)
    else:print(10**(n-1))
else:print(str(t)*n)

Another way is to start from the largest n-digit number, and "work backwards" to subtract the remainder:

n,t=input().split()
n=int(n)
t=int(t)
if t==10 and n==1:
    print(-1) # only case of -1
else:
    k=10**n-1 # largest number with n digits
    k=k-(k%t) # minus its remainder, leaving the quotient times t
    #         # thus k is now a multiple of t
    #         # k always has n digits as at most t-1<10 is subtracted from k
    #         # (think about when n=1, and think why t=10 is not possible in this case
    print(k)

In short, it is a problem of creativity that you get by problem soslving :)

Okay, since you're in 8th grade I don't think you would know about the two topics that I mentioned above.Keeping in mind this comment, if you're facing obstacles in solving Div2A level math problems, then here's the thing: those problems do not rely on the knowledge of some senior level math topic. Perhaps, they require you to think about the problem in a greedy way to figure out the solution. Sometimes it's about picking up a observation, and sometimes it's about coming up with a simple formula.

Though they're easy but you'll need to practice a good amount of such problems before you become good at them. So my advice would be to practice more of those problems (say around 50). And avoid wasting much time thinking on a single problem. Try brain-storming for say around 20 mins, and then go read the editorial if nothing clicks. This way you'll know about a lot of ideas/approaches which will help you solve these problems next time.