An ideal mono – atomic gas is trapped in a cylinder closed at its right end. The cylinder is divided into two parts by a fixed heat – conducting partition and a piston that is to the left of the fixed partition. The piston and walls of the cylinder donot conduct heat. Masses of the gas in the left and right parts are  $m_1$ and  $m_2$. A force F applied on the piston is slowly increased, starting from some initial value. The molar specific heat of the gas in the left part during this process is   $-nR$, Find n( Given  $\frac{m_2}{m_1}=2$)