How do I input a word, and shift it 6 letters? It just return 0 instantly

#include <windows.h>
#include <iostream>
#include <string>

using namespace std;

string encrypt (string cde);

int main()
{
    char A, B, C, letter, vault;
    string temp, ENC;
    start:
    cout << "Choose task: " << endl;
    cout << endl;
    cout << "A. Encrypt" << endl;
    cout << "B. Decrypt" << endl;
    cout << "C. Exit Program"<< endl;
    cout << endl;
    cin >> letter;
    system ("CLS");

    if  (letter == 'A' )
    {
        string cde, ENC;
        int shift;
        cout << "Please enter a message: ";
        getline(cin, cde);

        string temp = cde;
        int length;
        length = (int)temp.length();
        for (int i = 0; i < length; i++)
        {
            if (isalpha(temp[i]))
            {
                for (int j = 0; j < 6; j++)
                {
                    if (temp[i] == 'z')
                    {
                        temp[i] = 'a';
                    }
                    else if (temp[i] == 'Z')
                    {
                        temp[i] = 'A';
                    }
                    else
                    {
                        temp[i]++;
                    }
                }
            }
        }

        cout << "\nEncoded message: " << temp << endl;
    }


    else if (letter =='B' )
    {
        s2:
        cout << "B" << endl;
    }
    else if (letter == 'C')
    {
        s3:
        return 0;
    }

    else
    {
        cout << "Invalid Input.";
        Sleep(500);
        system ("CLS");
        goto start;
    }
    return 0;
}