Cryptographer's Conundrum Solution:

#include <iostream>
using namespace std;

int main() {
    string s;
    int total = 0;
    cin >> s;
    for (int i = 0; i < s.length(); i++){
        if (i%3 == 0 && s[i] != 'P')
            total++;
        if (i%3 == 1 && s[i] != 'E')
            total++;
        if (i%3 == 2 && s[i] != 'R')
           total++;
    }
    cout << total;
    return 0;
}