If the pipe is filled with helium at the same temperature, what fundamental frequency does it produce?

I assume that I just take (f/v1)(v2), with v1 being speed in air and v2 being speed in helium

this lead me to the equation:

(f*sqrt(M_air))/sqrt(M_He)

However, this is somehow off by a multiplicative factor. I can, however, find no way in which any other factor would be involved...