Fill in the missing event that causes deadlock in a database. There are two processes $($P$1$ and P$2),$ each of which needs to update two records $($R$1$ and R$2)$ and the following sequence leads to a deadlock:

$1.$ P$1$ accesses R$1$ and locks it$.$

$2.$ P$2$ accesses R$2$ and locks it$.$

$3.\_\_\_\_$

$4.$ P$2$ requests R$1,$ which is locked by P$1.$