Program that decodes a message in Fortran - fortran

Question

How can I take a text file that first lists a 2x2 matrix that was used to encrypt a message, followed by one integer per line (total 32 lines) that encode a message? Basically, I want to create a program that will:
1) Read and invert the 2x2 encryption matrix
2) Write a subroutine for determinant function to help invert matrix
3) Decrypt by reading in two integers at a time and inserting them into a character string (chracter*32 = full string)
And finally, print the hidden message.
I am pretty new to Fortran (and programming in general) but here is what I have so far and would appreciate any help.
program Program2
implicit none
INTEGER, DIMENSION(2,2) :: M, M2, M3
INTEGER :: B(32)
INTEGER :: row,col,max_rows,max_cols, Det, i, n, a
max_rows = 2
max_cols = 2
open(11, file = 'Data3.txt')
DO row = 1,max_rows
READ(11,*) (M(row,col),col=1,max_cols)
END DO
!Finding the inverse of a 2x2 matrix and reassigning.
M2(1,1) = M(2,2)
M2(2,2) = M(1,1)
M2(1,2) = -M(1,2)
M2(2,1) = -M(2,1)
! Could not get determinant function to yield correct answer (calc by hand)
M3 = M2/-1
print*, M3
print*, Det
open(11, file = 'Data3.txt')
i = a
do i = 1, 16
read(11,*) n
print*, n
enddo
close(11)
end program Program2
! Determinant function for a 2x2 matrix
function Determinant(M2) result(Det)
implicit none
INTEGER, DIMENSION(2,2) :: M, M2, M3
INTEGER :: B(32)
INTEGER :: row,col,max_rows,max_cols, Det, i, n, a
Det = M2(1,1)*M2(2,2) - M2(1,2)*M2(2,1)
end function Determinant
Here is the text file (or just a copy of what it looks like):
3 11
2 7
1488
955
703
458
1379
887
1465
943
1196
764
1392
895
1433
922
1403
902
1372
876
1467
943
697
454
1518
975
1596
1026
1392
895
1536
988
726
473

Answer 1

First of all, to get your Determinant function working, it needs a type (see Function has no implicit type):
INTEGER :: Determinant
Edit:
No need to be referenced as external, as pointed out in the comments. Thanks.
Then it can be used and it works well:
Det = Determinant(M2)
print*, Det
prints -1.
Second, could you please provide more explanation about what you want to do in step 3) so that we can help you out.
3) Decrypt by reading in two integers at a time and inserting them into a character string (chracter*32 = full string)