Align last element in Span to opposite side of box then first element

Question

I have a Span element with a index count, and an exit button in the same line. I need the exit button to be at the opposite side of the Span. However with my current CSS setup, all elements are spaced equally, how do I adjust this so it only affects the button?

  <span class="index-count">
      {{ documentInfoLabel() }} <b>({{ this.count }}/{{ this.filesCount }})</b>
        <b-button
        icon-left="window-close"
        size="is-small"
        @click="closePreview"
        >
        </b-button>
  </span>

 .index-count {
    display: flex;
    justify-content: space-between;
  }