program decay
      
      implicit none
      real time, tau, number, dt

      open(12,file='decay.dat')

      tau    = 5.0
      dt     = 1.0
      number = 100.0

      do time = dt, tau, dt
       number = number * (1.0 - dt/tau)
       write(12,*) time, number
      enddo

      close(12)

      end