# Change in momentum in Impulse of deformation and reformation

I was reading some text on coefficient of restitution when I encountered this ,

In the following you all would notice that in Impulse of deformation for particle $$m_2$$ we can see that change in momentum would be $$m_2v - m_2u_2 = m_2(v-u_2)$$ but what about particle $$m_1$$. When we write change in momentum for $$m_1$$ it should have been $$m_1v-m_1u_1$$ ie: $$m_1(v-u_1)$$ but then why is it written $$m_1(u_1-v)$$ . The same thing is happening in the Impulse of reformation part. I can't seem to identify where I am going wrong in my explanation. So can somebody please give me the right explanation.