What is the expiation for a woman who got married without knowing the obligation of observing the 'iddah?

If a woman is divorced before consummation, she has no waiting period ('iddah) and can remarry. However, if the divorce occurs after consummation, she must observe a waiting period of three menstrual cycles. If she marries before the 'iddah expires, her marriage is invalid by scholarly consensus. She must immediately separate from the second husband, complete her 'iddah from the first husband, then observe an 'iddah from the second husband, and then she may marry whomever she wishes. The second husband may marry her after her 'iddah from the first husband concludes, with a dowry, a guardian, and two witnesses.