program control_constructs
  implicit none
  integer :: i, n
  logical :: prime = .true.

  print *, 'Give integer: '
  read *, n
 
  sieve: do i = 2, n/2
    if (MOD(n,i)==0) then ! Remainder function MOD(n,i) = 
      prime = .false.     !                    n-INT(n/i)*i
      exit sieve
    end if
  end do sieve
  select case (prime)
  case (.true.)
    print *, n, ' is a prime number'
  case (.false.)
    print *, n, ' is not a prime'
  end select
end program control_constructs