My solution in python 3:

def decentNumber(n):
    if n % 3 == 0:
        print('5' * n)    
    elif n % 3 == 1 and n >= 10:
        print( '5'* (n - 10) + '3' * (10))
    elif n % 3 == 2 and n >= 5: 
        print('5' *(n - 5) + '3' * (5))
    else:
        print(-1)    
    return

When you divide by 3, you get remainder 0 or 1 or 2. If it 0, we can have a stirng of all 5's. If the remainder is 1, then if you subtract 10 ( which is 3 * 3 + 1) then the number is divisible by 3 -- however, number should be atleast 10. If remainer is 2, then if you subtract 5 (which is 3 *1 + 2) then the number is divisible by 3 -- however number should be at least 5. Otherwise, print -1.