Легко взломать! (шифровшик)

#include <iostream>
#include <conio.h>
#include <string>
#include <math.h>
#include <algorithm>
#include <vector>
#include <set>
#include <sstream>

using namespace std;

int main()
{
    string s, answer = "";

    cin >> s;

    int prom;
    if (s[0] >= 'A'&& s[0] <= 'Z')
    {
        prom = s[0] +5- 65;
        if (prom> 25)
            answer += (s[0] + 5) % 26+65;
        else
            answer += s[0] + 5;
    }
    else
    {
        prom = s[0]+5 - 97;
        if (prom > 25)
            answer += (s[0]+5)% 26+97;
        else
            answer += s[0] + 5;
    }
    for (int i = 1; i <= s.size() - 1; i++)
    {
        if (s[i] >= 'A'&& s[i] <= 'Z')
        {
            if (s[i] + prom > 90)
                answer += (s[i]-65 + prom) % 26+65;
            else
                answer += s[i] + prom;
            prom = s[i] - 65+prom;
        }
        else
        {
            if (s[i] + prom > 122)
                answer += (s[i]-97 + prom) % 26+97;
            else
                answer += s[i] + prom;
            prom = s[i] - 97+prom;
        }

    }

    cout << answer;
    _getch();
    return 0;
}