program main

  double precision error
  double precision f
  double precision f1
  double precision f2
  integer i
  integer j
  integer m
  integer n
  integer seed
  double precision stdev
  double precision sterr
  double precision value
  double precision x(2)

  m = 2
  n = 1000000
  seed = 123456789

  f1 = 0.0
  f2 = 0.0

  do i = 1, n  
    call random_number ( harvest = x(1:m) )
    value = f ( m, x )
    f1 = f1 + value
    f2 = f2 + value * value
  end do

  f1 = f1 / dble ( n )
  f2 = f2 / dble ( n - 1 )
  stdev = sqrt ( f2 - f1 * f1 )
  sterr = stdev / sqrt ( dble ( n ) )
  error = abs ( f1 - 1.0D+00 )

  write ( *, '(a)' ) ' '
  write ( *, '(a)' ) '      N          F           stdev   sterr      err'
  write ( *, '(a)' ) ' '
  write ( *, '(i7,2x,g14.6,2x,f10.4,2x,f10.4,2x,g8.2)' ) n, f1, stdev, sterr, error

  stop
end
function f ( m, x )

  integer m

  double precision f
  integer i
  double precision value
  double precision x(m)

  value = 1.0
  do i = 1, m 
    value = value * abs ( 4.0 * x(i) - 2.0 )
  end do

  f = value

  return
end