/in/foo.c: In function 'main':
/in/foo.c:19:30: warning: implicit declaration of function 'sqrt' [-Wimplicit-function-declaration]
   19 |         for (int i = 3; i <= sqrt(n); i += 2) {
      |                              ^~~~
/in/foo.c:2:1: note: include '<math.h>' or provide a declaration of 'sqrt'
    1 | #include <stdio.h>
  +++ |+#include <math.h>
    2 | #include <stdio.h>
/in/foo.c:19:30: warning: incompatible implicit declaration of built-in function 'sqrt' [-Wbuiltin-declaration-mismatch]
   19 |         for (int i = 3; i <= sqrt(n); i += 2) {
      |                              ^~~~
/in/foo.c:19:30: note: include '<math.h>' or provide a declaration of 'sqrt'