Conditional @HostBinding depending on @Input()

I'm trying to bind the CSS class foo to my host component, using @HostBinding depending on a test I do on a dynamic variable. But can't manage to make it work properly.
Here's what I already tried:

export class MyComponent {
    @Input()
    public input: string;

    @HostBinding('class.foo')
    public isFoo: Boolean = this.inputIsCorrect();

    constructor() {
    }

    private inputIsCorrect(){
        return (this.input != 'not correct');
    }
}

How I could make it work ? I was maybe thinking of way to listen to input changes ?