foo.cc: In function 'int main()':
foo.cc:9:6: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
    9 | scanf("%lf%lf%lf%lf", &x1,&y1,&x2,&y2);
      | ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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