elementary number theory - If $p=1cdot 3 cdot 5 cdot 7 cdot 9 cdot ... cdot 2011$, then the units digit of $p$ is five

I know there is a $5$ on the sequence, but i don't know how and why his presence leads to the final units digit of the product.

Answer

Hint: All odd numbers divisible by $5$ end in $5$.