smart variant

#include <iostream>
#include <string>
using namespace std;

int main()
{
    string alphabet = "abcdefghijklmnopqrstuvwxyz";
    string str;
    getline(cin, str);
    for (int i = 0; i < alphabet.size(); i++)
    {
        if (str.find(alphabet[i]) == string::npos && str.find(toupper(alphabet[i])) == string::npos)
        {
            cout << alphabet[i];
        }
    }
return 0;
}