To be divisible by both 2 and 3, it must be a number of the form
(since it needs no other prime factors to be as small as possible.)
When divided by two, the result is a perfect square:
So
is a perfect square.
Therefore m-1 and n are both even
Therefore m is odd and n is even.
When divided by three, the result is a perfect cube.
So
is a perfect cube.
Therefore m and n-1 are both multiples of 3.
Therefore m is a multiple of 3 and m is 1 more than a multiple of 3.
Putting everything together:
m is an odd multiple of 3.
The smallest such positive integer is 3, so m=3
n is even and 1 more than a multiple of 3.
The smallest such positive integer is 4, so n=4.






So there are 648 guests.
Checking:
648÷2 = 324 = 18²
648÷3 = 216 = 6³
Edwin