C언어 swea 2056 연원일 달력

https://swexpertacademy.com/main/code/problem/problemDetail.do?contestProbId=AV5QLkdKAz4DFAUq&categoryId=AV5QLkdKAz4DFAUq&categoryType=CODE

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#include <stdio.h>

int main(void) {
	int num;
	printf("숫자를 입력하세요: ");
	scanf_s("%d", &num);
	int year, month, day;
	year = num / 10000;
	month = (num % 10000) / 100;
	day = num % 100;

	switch (month) {
	case 1:
	case 3:
	case 5:
	case 7:
	case 8:
	case 10:
	case 12:
		if ((1 <= day) && (day <= 31)) {
			printf("%04d/%02d/%02d", year, month, day);
		}
		else
			printf("-1");
		break;
	case 4:
	case 6:
	case 9:
	case 11:
		if ((1 <= day) && (day <= 30)) {
			printf("%04d/%02d/%02d", year, month, day);
		}
		else
			printf("-1");
		break;
	case 2:
		if ((1 <= day) && (day <= 28)) {
			printf("%04d/%02d/%02d", year, month, day);
		}
		else
			printf("-1");
		break;
	default:
		printf("-1");
	}
}