program whileloop
  implicit none
  integer, parameter :: ikind=selected_real_kind(p=15)
  real (kind=ikind) :: sum,previoussum,x,smallnumber,error 
  integer :: i
  sum=0.0
  previoussum=0.0
  smallnumber=10.0_ikind**(-15.0)
  do i=1,1000
       x=i
       sum = sum + 1.0/(x**6)
       error=abs(sum-previoussum)
       if (error<smallnumber) then
            print *,'sum ',sum,' number of loops ',i
            exit
       end if
       previoussum=sum
  end do              
  end program whileloop