#include <assert.h>
#include <err.h>
#include <stdbool.h>
#include <stdio.h>

int nybble(char c) {
	if ('0' <= c && c <= '9') {
		return c - '0';
	} else if ('A' <= c && c <= 'F') {
		return c - 'A' + 10;
	} else if ('a' <= c && c <= 'f') {
		return c - 'a' + 10;
	} else {
		return -1;
	}
}

int main(void) {
	for (;;) {
		int high_nybble;
		bool eof_reached = false;
		for (;;) {
			char c = getchar();
			if (c == EOF) {
				if (feof(stdin)) {
					eof_reached = true;
					break;
				} else {
					err(1, "Error reading high nybble");
				}
			}

			high_nybble = nybble(c);
			if (high_nybble != -1) {
				break;
			}
		}

		if (eof_reached) {
			break;
		}

		int low_nybble;
		for (;;) {
			char c = getchar();
			if (c == EOF) {
				if (feof(stdin)) {
					errx(1, "Expected low nybble");
				} else {
					err(1, "Error reading low nybble");
				}
			}

			low_nybble = nybble(c);
			if (low_nybble != -1) {
				break;
			}
		}

		if (putchar((high_nybble << 4) | low_nybble) == EOF) {
			err(1, "Error writing byte");
		}
	}

	if (fflush(stdout) == EOF) {
		err(1, "Error flushing stdout");
	}

	return 0;
}