С++ перевод из римских

#include <iostream>
#include <cmath>

using namespace std;

void converter() {
    //переменные
    string line;
    char buff[100];
    char err[100];
    for (int i = 0; i < 100; i++) {
        buff[i] = NULL;
    }
    int chetdef = 0;

    int chetchik = 0;
    cout << "Введите ваше римское число\n -";
    cin.getline(buff, 100);

    for (int i = 0; i < 100; i++)
    {
        if (buff[i] == NULL) continue;

        switch (buff[i]) {
        case 'I':
            if (i + 1 < strlen(buff) && (buff[i + 1] == 'V' || buff[i + 1] == 'X' || buff[i + 1] == 'C')) {
                chetchik = chetchik - 1;
            }
            else if (i + 1 < strlen(buff) && (buff[i + 1] == 'I' && buff[i + 2] == 'I' && buff[i + 3] == 'I' )) {
                cout << "ERROR" << endl;
            }
            else chetchik = chetchik + 1;
            break;
        case 'V':
            if (i + 1 < strlen(buff) && (buff[i + 1] != 'V')) {

            }
            chetchik = chetchik + 5;

            break;
        case 'X':
            if (i + 1 < strlen(buff) && (buff[i + 1] == 'L' || buff[i + 1] == 'C' || buff[i + 1] == 'M')) {
                chetchik = chetchik - 10;
            }
            else if (i + 1 < strlen(buff) && (buff[i + 1] == 'X' && buff[i + 2] == 'X' && buff[i + 3] == 'X' && buff[i + 4] == 'X')) {
                cout << "ERROR" << endl;
            }
            else chetchik = chetchik + 10;
            break;
        case 'L':if (i + 1 < strlen(buff) && (buff[i + 1] == 'L')) {
            cout << "ERROR" << endl;
        }
                else chetchik = chetchik + 50;

            break;
        case 'C':
            if (i + 1 < strlen(buff) && (buff[i + 1] == 'D' || buff[i + 1] == 'M')) {
                chetchik = chetchik - 100;
            }
            else if (i + 1 < strlen(buff) && (buff[i + 1] == 'C' && buff[i + 2] == 'C' && buff[i + 3] == 'C' && buff[i + 4] == 'C')) {
                cout << "ERROR" << endl;
            }
            else chetchik = chetchik + 100;
            break;
        case 'D':
             if (i + 1 < strlen(buff) && (buff[i + 1] == 'D')) {
                cout << "ERROR" << endl;
            }
             else chetchik = chetchik + 500;

            break;
        case 'M':
            chetchik = chetchik + 1000;



            break;
        default: err[chetdef] = buff[i]; chetdef++;
            break;

        }
        // LL NO; VV NO ; CCCCC NO; DD NO;

    }
    cout << "Числа не прошедшие обработку:\n -";

    for (int i = 0; i < chetdef; i++) {
        if (chetdef > 0) {
            cout << err[i] << " ";
        }
        else cout << "Все числа прошли обработку";

    }
    cout << endl;
    if (chetchik == 0) {
        cout << "Вы не ввели число для перевода" << endl;
    }
    else {
        cout << "Ваше арабское число\n -" << chetchik << endl;
    }

}
int main() {
    setlocale(0, "rus");

    converter();

    system("pause");
    return 0;
}