Calculate the inverse of a matrix 19*19

[Santi Garcia Vallve]

I am trying to make my own programs with fortran77 to predict protein 
folding types by distances functions that make allowances for amino acid 
interactions (Chou and Zhang, 1994 JBC). I need to calculate the inverse 
of a matrix 19*19 (19 rows and 19 columns), how can I make it? It is 
possible to do it with fortran?
I think some calculators are able to do it, how does they do it?
I tried also to do it using Microsoft Excel 5.0 for mac, but I only 
obtain the first element (first number of the first row and column).
Any suggestion?......
Thank you

Santi Garcia Vallve
Biochemistry Dept.
URV  Catalonia  Spain
email:vallve at quimica.urv.es