Firstly i calculate the longest rope lenths at each tree.
Then just try to tether goats to every tree, calculating the square.
If sum of 2 maxrope are larger then lenght between trees:
double maxrad = std::max(maxr[i], maxr[j]);
double minrad = l - maxrad;
if (minrad < 1.0)
{
    maxrad = maxrad - (1 - minrad);
    minrad = 1.0;
}
if (maxrad >= 1.0)
maxsize = std::max(maxsize, square(maxrad)+sqree(minrad));

Try this test
4
0 0
5 0
6 0
6.5 0

Right answer is 53.4071

Thx, really nice test.
Now AC :)

Yeah... I was caught by that too :))

pretty nice test