\int_D {d\omega } = \int_{ - 1}^1 {\left( {\int_{ - \sqrt {1 - y^2 } }^{\sqrt {1 - y^2 } } {\frac{{\partial Q}}
{{\partial x}}dx} } \right)dy} - \int_{ - 1}^1 {\left( {\int_{ - \sqrt {1 - x^2 } }^{\sqrt {1 - x^2 } } {\frac{{\partial P}}
{{\partial y}}dy} } \right)dx} =