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Write a function P6.

• There are two objects A and B on a line, with a wall at the left. A is closer to the wall than B. Initially A and B are at rest, with masses 1 kg and $n$ kg. B suddenly moves toward A at a speed of 1 m/s and collides. The collision is elastic.
• input parameter: a positive integer $n$ ($n \le 10^6$)
• return value: the number of collisions between A and B or between A and the wall
• Hint: When two objects with masses $m\_1$ and $m\_2$ collide with initial velocities (not speeds) $v\_{1i}$ and $v\_{2i}$, the final velocities $v\_{1f}$ and $v\_{2f}$ satisfy $m\_{1}v\_{1i} + m\_{2}v\_{2i} = m\_{1}v\_{1f} + m\_{2}v\_{2f}$. In an elastic collision, kinetic energy is conserved.