Help with calculating errors and slope in Fortran 90.

Writing a fortran program to find errors in a and b?
I am writing a program to calculate the y-intercept, best slope and the errors in a and b. I have been able to write up until the errors, which is where im struggling. This is what i've done so far:

PROGRAM assignment4
IMPLICIT NONE
INTEGER :: i,n
REAL :: sumx, sumy, sumxx, sumxy, a, b, e, f
REAL :: sumax, sumb, sumyi, sumd
REAL :: x(100),y(100)
!
PRINT*,"How many points to be entered(Max 100)?"
READ*,n
PRINT*,"Enter the data pairs,x,y"
!
DO i=1,n
READ*,x(i),y(i)
ENDDO
!
Sumx=0
Sumy=0
Sumxx=0
Sumxy=0
!
DO i=1,n
Sumx= sumx+x(i)
Sumy= sumy+y(i)
Sumxx= sumxx+x(i)**2
Sumxy= sumxy+x(i)*y(i)
ENDDO
!
a=(sumxy-sumx*sumy/n)/(sumxx-(sumx*sum…
b=sumy/n-a*sumx/n
PRINT*,"Slop is:",a
PRINT*,"Intercept is:",b
!
Sumx=0
Sumy=0
Sumxx=0
Sumxy=0
!
DO i=1,n
Sumy= suma*x(i)-y(i))**2
Sumx= (sumx+x(i)**2)
Sums= (((sumx+x(i))*a)+b-(y(i)))**2
ENDDO
!
e=SQRT((Sumax)/((n-2)*Sumd))
f=SQRT((Sumax)*(Sumx**2))/(n*(n-2)*Sumd)
!
PRINT*,"Error in a is:",e
PRINT*,"Error in b is:",f


END PROGRAM assignment4


I am given that delta a^2 = S/(n-2)D and delta b^2= S*sum(x(i)^2) / n(n-2)D

S=sum of a*x(i)+b-y(i) all squared
D = sum x(i)^2 - n*x^-2

Any help will be appreciated thanks!