It's actually 1+2f versus 1-f. (I agree that the wording is not very clear. I misinterpreted it too at first and was very confused at how my solution was being

It's actually 1+2f versus 1-f. (I agree that the wording is not very clear. I misinterpreted it too at first and was very confused at how my solution was being rejected. If you look at the short worked example they give, though, it's unambiguous.) You're using the normal approximation to the binomial distribution. It may or may not be a good enough approximation. Better to do the binomial thing exactly. This requires arbitrary-precision integer arithmetic, but that's pretty easily available.