We have a n-dimentional hypersurface embedded in the n+1 Eucledean space. We know that this hypersurface is compact without boundary, convex (not necessarly strictly convex), and that the k-th symmetric polynomial in the principal curvatures is constant. Than can I deduce that this hypersurface is a sphere?

Note: in the case k=1 (maybe in the case k=n ?) the answer is "yes".