Constant Integer pointer passed to the function expecting a integer pointer modifies the value of constant integer?

Question

I am trying to learn the constant integer pointer and it's behavior;

#include<stdio.h>
void test1(int*); 
int main()
{
    const int b=20;
    const int *q; 
    q=&b;
    test1(q);       //compiler will give warning
    printf("value of b=%d\n",b); // here modified value of b is printing  
}

void test1(int *ptr)
{
    *ptr=50;           // HERE constant integer in the main is modified , but how?  
}

In the above snippet I am intentionally passing the constant integer pointer to the function that expects an integer pointer argument, what I am unable to comprehend is how is value of the variable "b" which is constant integer is updating, even being a constant integer?