This problem is intended to be a simple warm-up, with a very fixed
format that eliminates any need to do any parsing.  Simply examine
the zeroth, fourth, and eighth characters and check the arithmetic.

if ((s[0] - '0') + (s[4] - '0') == (s[8] - '0'))
   cout << "YES" << endl ;
else
   cout << "NO" << endl ;

A cleaner solution is to parse it using the scanning functionality
in most programming languages:

int x, y, z ;
char op, equal ;
cin >> x >> op >> y >> equal >> z ;
cout << ((x + y == z) ? "YES" : "NO") << endl ;