Erlang 28 — run it in a Firecracker microVM on ls-judge. Real toolchain, no wrapper.
Clicking "Run now" opens the playground with this exact code and a matching test case — passes on first Run.
-module(solution).
-export([main/0]).
main() ->
{ok, [N]} = io:fread("", "~d"),
io:format("~b~n", [N * 2]).
21
42
Every submission is compiled and executed inside its own Firecracker microVM — the same isolation primitive AWS Lambda uses. No shared container, no shared kernel with other users' code. The Erlang 28toolchain shipped with the microVM is the real thing (not a browser-side emulation): syscalls behave the way they do in production, and the compile / runtime behaviour matches what you would see on your own machine with that version installed.
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