In German schools, high school scores convert to lower class' scores as follows:

pts score in words
15  1+ 
14  1  very good
13  1-
12  2+
:        etc.
04  4-
03  5+
02  5  poor (failure)
01  5-
00  6  nothing at all (failure)

There is no 6+, only a 6; but the difference from 5- to 6 is 1 point, not 2 as expected. 

=LEFT((18-A1)/3)&MID("+-",MOD(A1-(A1=0),3)+1,1) converts points to a score

=MAX(61-3*LEFT(A1)-CODE(MID(A1&",",2,1)),) converts a score to points (formula by Bernd Plumhoff)