foo.cc: In function 'int main()':
foo.cc:6:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
    6 |  scanf("%lf%lf%lf%lf",&xa,&ya,&xb,&yb);
      |  ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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