Why output -1 in find_first_of?

Question

C++ / C#

Hello. I can't figure out what is the error in find_first_of? When compiled, it outputs -1, although the condition seems to be set correctly.

...
int num = 0;
  string s = "";
  string a = "";
  getline(fin, s);
  getline(fin, a);
  while (s != "") {
    if (s.find(str, 0) != 1) {
      string konec = ".!?";
      num = a.find_first_of(konec, 0);
      cout << num;
      string sub = a.substr(0, num);
      if (sub.find(str, 0)) {
        cout << sub << endl;
      }
      a.erase(0, num);
      s.erase(0, num);
    }
  }
...