Constant reference can be changed?

Question

Hello

Output of below code is 1111 , but I was expecting compile error :

int a =10;
int const &b = a;
a = 11;
cout << a << b;

If we refer second line, b is constant and it is just an alliance of variable a... now, third line is trying to change value of a (inturn variable b). how come value is printed instead of compile error.