The decoder is also a RNN which generates the target sequence Y based on X, and predicts the next label y t based on the context vector c t and all previous labels [y 1 , y 2 , ..., y t−1 ] predicted by the  same decoder.
the joint probability of the target sequence is defined as: where .., y t−1 ] and the conditional probability of label y t can be modeled by the decoder, and defined as: where is the target vocabulary size, and s d is the hidden state size of decoder.