constant qualifier

this function is giving value 10 after modification in main but when printed in fun(), it is giving modified value.. how is this happening??

#include<stdio.h>

#include<conio.h>

void fun(const int *s)
{
printf("In this function %d\n",*s);
}

int main()

{

const int a = 10;

int *p = (int*)&a;

printf("before modification a=%d\n",a);

*(int*)&a = 12;

printf("after modification a=%d\n",a);
fun(&a);
getch();

return 0;

}