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

#define N 20
int main() {
	char str[N+1];
	char chars[] = "ACGT";

	int ii;
	for (ii = 0; ii < N; ++ii) {
		int x = rand() % strlen(chars);
		/*switch (x) {
		case 0:
			str[ii] = 'A';
			break;
		case 1:
			str[ii] = 'C';
			break;
		}*/
		str[ii] = chars[x];
	}
	str[ii] = '\0';
	puts(str);

	return 0;
}