In JavaScript:

console.log(arr, n) gives me:
[ 1, 2, 3, 4 ] 2

and result of my function gives:
9

what I got from the system is:
expected undefined to equal 9

Aren't we supposed to return either equation result or -1?

in C, sample tests won't compile:

"/tmp/fixture-4fb33b.o: In function the_multiply_function_should_pass_all_the_tests_provided_impl': fixture.c:(.text+0x32): undefined reference to foo'
clang: error: linker command failed with exit code 1 (use -v to see invocation)"

Same program works when submitted.