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Write a function P6:

• There are two objects A, B on a line and a wall is placed at left. A is closer than B to the wall. Initially, A and B are static and each has 1 kg, $n$ kg. Suddenly, B moves toward A with speed 1 m/s and collides. The collision is elastic.
• input parameter: a positive integer $n$ ($n \le 10^6$)
• return value: the collision number between A and B or A and wall
• Hint: When two objects with mass $m\_1$ and $m\_2$ collide with the initial velocity (not speed) $v\_{1i}$ and $v\_{2i}$, the final velocity $v\_{1f}$ and $v\_{2f}$ satisfy the equation $m\_{1}v\_{1i} + m\_{2}v\_{2i} = m\_{1}v\_{1f} + m\_{2}v\_{2f}$. And in elastic collision, the kinetic energy is conserved.