Starting from my second attempt (first was WA) I am always getting "Checker failed" independently of the code. Even, if I submit the first attempt's code, which was WA.

I see that my submissions were tested. But there is still a problem: I suspect, that checker is not correct, as it rejected another optimal answer.
Compare submissions 10791903 and 10791934 : they have only difference in "<" / "<=" in "if (d[..] < answer) { ..update answer..}". One of them is AC, another one is WA40.