/in/foo.c: In function 'main':
/in/foo.c:11:11: warning: implicit declaration of function 'abs' [-Wimplicit-function-declaration]
   11 |         t=abs(z1-z2);
      |           ^~~
/in/foo.c:3:1: note: include '<stdlib.h>' or provide a declaration of 'abs'
    2 | #include <math.h>
  +++ |+#include <stdlib.h>
    3 | int main()