Two rigid boxes containing different ideal gases are placed on...

Two rigid boxes containing different ideal gases are placed on table. Box A contains one mole of nitrogen at temperature $T_0$, while box $B$ contains 1 mole of helium at temperature $7 / 3 T_0$. The boxes are then put into thermal contact with each other and heat flows between them until the gases reach a common final temperature (ignore the heat capacity of boxes) then the final temperature of gases, $T_f$ in terms of $T_0$ is