chopped-off corners from a button with a solid border

I want to cut two corners of a button with a 3px solid border and no background, like the example below, preferably with only CSS. I tried many options and searched the web, but no luck so far... Do you guys know a way to achieve this? Thanks for any response!

What I got so far is:

a.button{
    padding: 10px 18px;
    border: 3px solid $blue;
    text-transform: uppercase;
    font-size: 15px;
    border-top-left-radius: 15px;
    border-bottom-right-radius: 15px;
}

The problem is that this results in a radius corner, and I want the corners to be a straight cut.