/in/foo.c: In function 'main':
/in/foo.c:8:27: warning: implicit declaration of function 'sqrt' [-Wimplicit-function-declaration]
  for (int i = 2; i < (int)sqrt((double)number); i++) {
                           ^~~~
/in/foo.c:8:27: warning: incompatible implicit declaration of built-in function 'sqrt'
/in/foo.c:8:27: note: include '<math.h>' or provide a declaration of 'sqrt'