I need help with a formula...

I have two columns of data and I need to figure out a way to have Excel
figure out the third column. The data is being imported into a database and
will be creating a tree structure.

The two columns I have are Level (indicates which level in the tree
structure this record needs to be at) and number (a unique number for this
record).

I need a formula (if there is one) that will determine the Parent Record.
So in the example below, I have filled out the Parent Code for several of the
records. The parent for "3: M3.A.1" is "3: M3.A". Is there a way to have a
lookup or vlookup do this work for me?

Level Number Parent Code
1 3: M3.A
2 3: M3.A.1 3: M3.A
3 3: M3.A.1.1 3: M3.A.1
4 3: M3.A.1.1.1 3: M3.A.1.1
4 3: M3.A.1.1.2 3: M3.A.1.1
4 3: M3.A.1.1.3 3: M3.A.1.1
4 3: M3.A.1.1.4 3: M3.A.1.1
4 3: M3.A.1.1.5 3: M3.A.1.1
3 3: M3.A.1.2
4 3: M3.A.1.2.1
4 3: M3.A.1.2.2
3 3: M3.A.1.3
4 3: M3.A.1.3.1
4 3: M3.A.1.3.2
4 3: M3.A.1.3.3
2 3: M3.A.2
3 3: M3.A.2.1
4 3: M3.A.2.1.1
4 3: M3.A.2.1.2
4 3: M3.A.2.1.3