/in/foo.c: In function 'main':
/in/foo.c:7:13: warning: implicit declaration of function 'sqrt' [-Wimplicit-function-declaration]
  for(i=2;i<=sqrt(n);i++)
             ^~~~
/in/foo.c:7:13: warning: incompatible implicit declaration of built-in function 'sqrt'
/in/foo.c:7:13: note: include '<math.h>' or provide a declaration of 'sqrt'