program main
implicit none
integer*8,parameter :: num_step=500000000
integer*8 :: i
real(kind=8) :: sum,step,pi,x
real(kind=8) :: stime,etime,rtc
step = (1.0d0/dble(num_step))
sum = 0.0d0
write(*,400)
stime=rtc() !starting time
do i=1,num_step
x=(dble(i)-0.5d0)*step
sum=sum+4.d0/(1.d0+x*x) !F(x)
enddo
etime=rtc() !ending time
pi=step*sum
write(*,100) pi,dabs(dacos(-1.0d0)-pi)
write(*,300) etime-stime
write(*,400)
100 format(' PI = ', F17.15,' (Error =',E11.5,')')
300 format(' Elapsed Time = ',F8.3,' [sec] ')
400 format('---------------------------------------')
stop
end program
