foo.c: In function 'main':
foo.c:13:12: warning: unused variable 'd' [-Wunused-variable]
     double d, a[100];
            ^
foo.c:14:5: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d", &n);
     ^~~~~~~~~~~~~~~
foo.c:17:9: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
         scanf("%lf%lf%lf%lf", &x_1, &y_1, &x_2, &y_2); //
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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