Doesn't compiler give an ambiguous error for const and not-const functions

I have overload const and non-const functions in c++ structure. Then, I have run the program and I wondered, it's working fine without ambiguous error.

#include <iostream>

struct St 
{
  int f() const 
  { 
      return 1; 
  }

  int f()
  { 
      return 2;
  }
} s;

int main() 
{
    int ret = s.f();  
    std::cout<<ret<<std::endl;
    return 0;
}

So, I just want to know, Why doesn't compiler give an ambiguous error?