Constraint relation 03

Classical Mechanics Level 3

Consider the situation shown in figure. Both the pulleys and the string are light and all the surfaces are smooth. The tension in the string connecting block of mass 1 kg and pulley is $\dfrac ab$ where $a$ and $b$ are coprime positive integers. Find the value of $a-b$ .

Take $g=10\text{ m/s}^2$ .