program rsa
  implicit none
  integer :: n, m

  print *, "Enter numbers to compute gcd:"
  read(*,*) n, m

  print *, "gcd: ", gcd(n,m)

  contains

    recursive function gcd(n,m) result(x)
      implicit none
      integer, intent(in) :: n,m
      integer :: x

      if (n == 0) then
        x = m
      else if (m == 0) then
        x = n
      else
        x = gcd(m, mod(n,m))
      end if
    end function gcd

end program rsa