foo.c: In function 'main':
foo.c:10:5: warning: ignoring return value of 'freopen', declared with attribute warn_unused_result [-Wunused-result]
     freopen("sticks.in", "r", stdin);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.c:11:5: warning: ignoring return value of 'freopen', declared with attribute warn_unused_result [-Wunused-result]
     freopen("sticks.out", "w", stdout);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.c:13:5: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d", &n);  //读入火柴棒数量。
     ^~~~~~~~~~~~~~~

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