collision problem- can i get help please ?

i have a problem, i need some explanation for how to fix it, can i get some help please ?
full problem:
https://drive.google.com/file/d/1yV5Es5CySy5u_mr1cEyoH6LdOLQjyP6E/view?usp=sharing

collision problem:
there are n particles numbered from 0 to n-1 lined up from smallest to largest ID along the x-axis.
the particles are all released simultaneously. once released, each particle travels indefinitely in a straight line along the positive x-axis at a speed.
when two particles collide, the faster particle moves through the slower particles and they both continue moving without changing speed or direction.
write a function that return the number of collision occurring with particle pos.
collision has the following parameters:
speed[speed[0] … speed[n-1]]: an array of speed* indication speed of particle i.
pos: index of the particle for which to count collisions.
constraints

  • 1 <= n <= 100000
  • 1 <= speed* <= 1000000000
  • 0 <= pos< n

input:
8
6
1
6
3
4
6
8
2

output:
2

explanation:
the particle at pos = 2 will collide with particle 0 and 1 as they pass it from behind.