exiting loops on condition

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