iv><div class="row"><div class="column"><h4 class="ui top attached block header">输出格式</h4><div class="ui bottom attached segment font-content"><div style="position: relative; overflow: hidden;"><div class=content>应仅有一行，该行给出一个实数E(R)，表示圆环的“美观程度”的期望值。
</div></div></div></div></div><div class="row"><div class="column"><h4 class="ui top attached block header">样例</h4><div class="ui bottom attached segment font-content"><div style="position