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<div><span style="font-size: medium">Output one integer number&nbsp;&mdash; the sought number of pairs of friends.</span></div></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: relative; overflow: hidden;"><h4>样例输入</h4><div class="ui existing 