#include <stdio.h>
#include <string.h>

char res[100];

char * convert(const char * str) {
    size_t n = strlen(str);
    int iter = (int) n - 1;
    int counter = 0;

    int sz = -1;
    char get[16];
    for (char i = 0; i < 10; ++i) {
        get[i] = ('0' + i);
    }
    for (char i = 0; i < 6; ++i) {
        get[10 + i] = ('A' + i);
    }
    size_t size = (n + 3) / 4;
    // char res[size];
    int cur = 0;
    int base = 1;
    char tmp = 0;
    while (iter >= 0) {
        if (counter == 4) {
            counter = 0;
            res[size - cur - 1] = get[tmp];
            base = 1;
            tmp = 0;
            cur++;
        }
        tmp += base * (str[iter] - '0');
        base *= 2;
        counter++;
        iter--;

    }
    if (counter > 0) {
        res[size - cur - 1] = get[tmp];
    }


    return res;

}