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



#define IO_ERROR 3
#define DEPENDENCY_LOOP 2
#define BAD_INPUT_FORMAT 1
int main(int argc, char *argv[]) {

	static uint32_t prereq[26] = {0};
	uint32_t incomplete = 0;
	int error;

	/* Grab all the input rules. */

	{
		FILE *input;
		error = IO_ERROR;
		if (input = fopen(argc > 1? argv[1]: "input.txt", "r")) {
			#define MAX_INPUT_WIDTH 50
			static char buf[MAX_INPUT_WIDTH];
			error = 0;
			while (fgets(buf, MAX_INPUT_WIDTH, input)) {
				char pre, step;
				static char format[] =
					"Step %c must be finished before step %c can begin.";
				error = BAD_INPUT_FORMAT;
				if (
					buf[strlen(buf) - 1] == '\n' &&
					sscanf(buf, format, &pre, &step) == 2 &&
					'A' <= pre && pre <= 'Z' &&
					'A' <= step && step <= 'Z'
				) {
					error = 0;
					prereq[step - 'A'] |= 1 << pre - 'A';
					incomplete |= 1 << pre - 'A' | 1 << step - 'A';
				}
				if (error) break;
			}
			fclose(input);
		}
		if (error) return error;
	}

	/* Keep running the first ready task until either they're
	 * all complete or progress is blocked by a dependency loop. */

	while (incomplete) {
		uint32_t progress, taskbit;
		int task;
		progress = 0;
		for (task = 0, taskbit = 1; task < 26; ++task, taskbit <<= 1) {
			if ((taskbit & incomplete) && !(prereq[task] & incomplete)) {
				putchar(task + 'A');
				progress |= taskbit;
				incomplete &= ~taskbit;
				break;
			}
		}
		if (!progress) {
			error = DEPENDENCY_LOOP;
			break;
		}
	}

	putchar('\n');
	return error;
}