At first I had no ideas how to solve it ))
Then I thought that it is a completely physical problem and started to write formulas....
When, after about an hour, I came to integrals I understood that........ I am not a physicist... but I am a PROGRAMER! And at the same moment a good idea came to me: "Oh my God, It is a kind of a labyrinth!"
In 10 minutes source was done and I got AC. Now I am thinking... how easier to be a programmer, than physicist :)))

Oh, yes... I had like story. :)))
But physical solution is very easy :)
It's parabola, I don't remember formula but it is
something like y = m*(x^2)/(n^2)
So we can easly solve it O(n) using DP.

I think, if you now some physics you can write simple DP, using energy saving rule.

plz post your AC code
or send it to me
cpp_student@163.com