\frac{d E}{d \alpha_m} = \sum_{y_i \neq k_m(x_i)} w_i^{(m)}e^{\alpha_m} - \sum_{y_i = k_m(x_i)} w_i^{(m)}e^{-\alpha_m} .
\alpha_m = \frac{1}{2}\ln\left(\frac{\sum_{y_i = k_m(x_i)} w_i^{(m)}}{\sum_{y_i \neq k_m(x_i)} w_i^{(m)}}\right) .