INTEGER ::  n0, t, nt, k, nk
REAL :: r, w, p = 5.0/6.0
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!
   n0 = 1000
   OPEN(8, FILE='data1000')
   OPEN(9, FILE='decay1000')
!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
   w = 10.0/n0
   WRITE(8, '(I3, F6.2)')  0, n0*w
!
   nt = n0
   CALL RANDOM_SEED     ! 乱数列発生の「種」（初期値）
!
DO t=1, 10
!
! ここはベクトル化しなければならないが，内容は同じことだ。
   nk = nt
   DO k=1, nk
      CALL RANDOM_NUMBER(r)     !  (0,1) 間の一様乱数
      IF(r >= p) nt = nt-1
   END DO
!
   WRITE(8, '(I3, F6.2)')  t, nt*w
   WRITE(9, '(I3, F6.2)')  t, (nk-nt)*w
END DO
!
   CLOSE(8)
   CLOSE(9)
END