As I have learnt in my lecture, the proof to Sard's theorem at the last step has shown if $k>\frac{n}{m}-1$ the $f(C_k)$ has zero measure in $M$. But the statement of Sard's theorem said $k\geqslant n-m+1$. $C_k$ represents the set of all derivatives till $k$-th order are vanishing.
How does it come?