int main()
{
	int n;
	char input[501];
	scanf("%d", &n);
	gets(input);
	gets(input);
	int len = strlen(input);

	int count[501] = {0};//????i???i?????n???????????????count[i]=-1??????????????
	int i, j, k;
	for(i = 0; i < len; i++) {
		if(i + n > len) {
			break;//???????????n??????
		}
		if(count[i] == -1) {
			continue;//count[i]==-1????????????
		}
		for(j = i + 1; j < len; j++) {
			if(j + n > len) {
				break;//????????????????n??????????
			}
			int equal = 1;//equal==1?????????equal=0??????
			for(k = 0; k < n; k++) {
				if(input[i+k] != input[j+k]) {
					equal = 0;
					break;
				}
			}

			if(equal == 1) {
				count[i]++;//??count??
				count[j] = -1;//??????
			}
		}
	}

	//???????
	int max = -1, max_pos = 0;
	for(i = 0; i < len; i++) {
		if(count[i] > max) {
			max = count[i];
			max_pos = i;
		}
	}
	//?????????????????
	if(max + 1 <= 1) {
		printf("NO\n");
		return 0;
	}
	printf("%d\n", max+1);
	for(i = 0; i < len; i++) {
		if(count[i] == max) {
			for(j = i; j < i + n; j++) {
				printf("%c", input[j]);
			}
			printf("\n");
		}
	}
	

}