change border radius of mat-flat-button on angular

I am developing an angular app and would like to change the border radius of mat-flat-button. I am able to change the background color of the button using css, but not the border radius. my code

<div flexLayout = "row">
<button class="flat_button" (click)="toggle_color()" mat-flat-button>B2-L1</button>
<button mat-flat-button>B2-L3</button>
<button mat-flat-button>B2-L3</button>
<button mat-flat-button>B2-L4</button>

</div>


.css

.mat-flat-button {
background-color: grey;
border-radius: 0 px;
}

.mat-button {
border-radius: 0 px;
}


can someone help me with this?

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