x_{21} = \left(x_{18} + x_{7} + x_{19} + x_{8}\right) / 2\\
x_{22} = \left(x_{18} + x_{7}\right) \cdot k_{16}\\
x_{23} = \left(x_{19} + x_{8}\right) \cdot k_{17}\\
x_{24} = \left(x_{7} + x_{8}\right) / 2 \cdot k_{18}\\
x_{25} = x_{7} \cdot k_{19}\\
x_{26} = x_{8} \cdot k_{20}\\
x_{27} = x_{20}\\
x_{28} = \left(x_{19} + x_{8}\right) / \left(x_{18} + x_{7}\right)