How to do the same calculation for multiple .dat files - fortran

Question

I want to do a fixed calculation step for multiple .dat files.
Here is my code for what I want to do with one .dat file i.e. the calculation:
dimension t(128716),x(128716)
open (unit=88,file='ALFA-gua-100m-2.dat',status='unknown')
do i=1,128716
read(88,*)t(i),x(i)
enddo
sum=0
do j=1,128716
sum=sum+x(j)
enddo
write(*,*)sum/128716
close(88)
stop
end
How do I go about this? Please suggest!
Here is my code for multiple file :
dimension t(128716),x(128716)
open (unit=11,file='ALFA-gua-100m-2.dat',status='unknown')
open (unit=12,file='ALFA-gua-100m-5.dat',status='unknown')
do i=1,2
ii = i + 10
do j=1,128716
read(ii,*)t(j),x(j)
enddo
sum=0
do k=1,128716
sum=sum+x(k)
enddo
enddo
do l=1,2
ll = l + 10
write(ll,*)sum/128716.0
close(ll)
enddo
stop
end
But its not working.

Answer 1

An addendum to #VladimirF's answer.
To sum all the elements in an array called x we can simply write
sumx = sum(x)
there is no need for the programmer to write a loop at all. If using an array of sums, then something like
sums(1) = sum(x)
would be appropriate.
Then to calculate the mean of an array I'd write
meanx = sum(x)/size(x)
While I'm writing: it's not a good idea to call a variable sum. There's an existing intrinsic function of that name and it will only confuse readers (though not the compiler) to have a variable of that name too.

Answer 2

You cannot use the same sum for two iterations of the i loop when you have two separate loops. It will get overwritten when processing the second file.
You can join the loops into one.
do i=1,2
ii = i + 10
do j=1,128716
read(ii,*)t(j),x(j)
enddo
sum=0
do k=1,128716
sum=sum+x(k)
enddo
write(ii,*)sum/128716.0
close(ii)
enddo
You can use an array for the sums sums(i).
do i=1,2
ii = i + 10
do j=1,128716
read(ii,*)t(j),x(j)
enddo
sum=0
do k=1,128716
sums(i)=sums(i)+x(k)
enddo
enddo
do l=1,2
ll = l + 10
write(ll,*)sums(l)/128716.0
close(ll)
enddo