At Least One Check Box Required

I have a checkbox group. I would like to make it so that at least one box has to be selected. How can I do that?

<div class="form-group" id="check_special_interest" is="dmx-checkbox-group">
                                                <legend class="col col-form-label">Special Interest</legend>
                                                <div class="form-check">
                                                    <input class="form-check-input" type="checkbox" value="Reg" id="si-reg" name="si-reg">
                                                    <label class="form-check-label" for="si-reg">Regular</label>
                                                </div>
                                                <div class="form-check">
                                                    <input class="form-check-input" type="checkbox" value="Y" id="si-youth" name="si-youth">
                                                    <label class="form-check-label" for="isi-youth">Youth</label>
                                                </div>
                                                <div class="form-check">
                                                    <input class="form-check-input" type="checkbox" value="MW" id="si-mw" name="si-mw">
                                                    <label class="form-check-label" for="si-mw">Mature Worker</label>
                                                </div>
                                                <div class="form-check">
                                                    <input class="form-check-input" type="checkbox" value="OW" id="si-ow" name="si-ow">
                                                    <label class="form-check-label" for="si-ow">Older Worker</label>
                                                </div>
                                                <div class="form-check">
                                                    <input class="form-check-input" type="checkbox" value="I" id="si-ind" name="si-ind">
                                                    <label class="form-check-label" for="si-ind">Indiginous</label>
                                                </div>
                                                <div class="form-check">
                                                    <input class="form-check-input" type="checkbox" value="IM" id="si-im" name="si-im">
                                                    <label class="form-check-label" for="si-im">IM?</label>
                                                </div>
                                            </div>