How to do calculations using string!

As already suggested you may convert the CString variable into a numeric one, before performing the subtraction (and then convert the result back to a CString in order to display it). However you should be also able to track down where the CString originates and modify the program (that is you should store numeric values in numeric variables and convert them to strings only when you need to represent them).

An alternative to sscanf is to use std::stringstream. However you need to convert CString to std::string and vice versa

Here a snippet from Codeguru how to do just that, and then I'll tell you how to use StringStream

CString cs ("Hello");

// Convert a TCHAR string to a LPCSTR
CT2CA pszConvertedAnsiString (cs);
// construct a std::string using the LPCSTR input
std::string strStd (pszConvertedAnsiString);

And now comes the stringstream part. Here's how I would do it.
//here's the std::string
std::string ss("10");

//use the string as an input stream
istringstream astream(ss,ios::in);

int val;

//extract the value from the input stream
astream>>val;
val -= 1;

//now use the string as an output stream
ostringstream os(ios::out);

//extract the value
os<<val;
//create a string out of it
string ass = os.str();
//convert your std::string to a CString
CString final(ass.c_str());

However you should be also able to track down where the CString originates and modify the program (that is you should store numeric values in numeric variables and convert them to strings only when you need to represent them).