C++ avoid arithmetic operation on size_t time

Here is a simple test code about this topic

#include <typeinfo>

    string s= "a";
    int i = 0;
    cout<<i<<" "<<typeid(i).name()<<endl;
    cout<<s.length()<<" "<<typeid(s.length()).name()<<endl;
    cout<<i - s.length()<<" "<<typeid(i - s.length()).name()<<endl;

The output is a very large number (18446744073709551615) instead of -1 as intended (see below).

0 i
1 m
18446744073709551615 m

As the type of s.length() is size_t. size_t is unsigned int or unsigned long depending on the machine used. It seems that the compiler converts the value to an unsigned long type.
To avoid this kind of problem, using something int n = s.length(); and then use this variable to do the calculations.