Negating the css property of one class with another

Negating the css property of one class with another

I have this:
<div class="style1 neg">
<input type="submit" value="blah" />
</div>
And the css:
.style1 {
float: left;
margin-top: 25px;
}
.style1 .neg {
margin-top: 0;
}
I'm trying to negate margin-top, but when I inspect the element in the
browser, the 'neg' style seems to be ignored completely. How would I do
this properly?