birthday probability program in Fortran 90

Question

I have been given an assignment to create a Fortran 90 program that calculates how many people are needed so that the probability of two or more people having the same birthday becomes 90%.

I would like to define variables as follows:

• P2 = probability of having at least 2 people of the same birthday among n people
• P1 = probability that all n people have different birthdays
• P2= 1- P1
• P1 = (364/365)(363/365)(362/365).... ((365-n-1)/365)

Edited: This is the code I have tried to compose:

program birth1
  implicit none
  integer:: n
  real(8):: P1, P2

n=1
P1 = 1

     do
        p1 = P1 * ((365.0d0 -n +1)/365.0d0)

        If (P1 <= 0.1) exit

         n= n+1
   end do

  write (*,*) n
  stop
  end program birth1

This code has worked and I got 41 as the output. Thank you.