C Language Example 3--enter a certain date of the year, to determine the day is the first days of the year?

question: Enter a certain date of the year, Judge this day is the first days of the year.

   /* Procedure analysis: Take March 5 as an example, should first add up the first two months,
   and then add 5 days that the first days of this year, special circumstances, leap years and enter a
   month greater than 3 o'clock to consider an additional day.
* *
#include <stdio.h>
int main () {
	int day, month, year, sum, Leap;
	printf ("\nplease input year,month,day\n");
	scanf ("%d%d%d", &year,&month,&day);
	Switch (month) {   //first calculates the total number of days before month of month 
		1:sum = 0;break;
		Case 2:sum = 31;break;
		Case 3:sum = 59;break;
		Case 4:sum = 90;break;
		Case 5:sum = 120;break;
		Case 6:sum = 151;break;
		Case 7:sum = 181;break;
		Case 8:sum = 212;break;
		Case 9:sum = 243;break;
		Case 10:sum = 273;break;
		Case 11:sum = 304;break;
		Case 12:sum = 334;break;
		default:printf ("Data error!"); break;
	sum = sum + day;    Plus days of the day 
	if (year%400==0| | (year%4==0&&year%100!=0)) {  //Judge is leap year 
		leap = 1;
	} else{
		leap = 0;
	}
	if (leap = = 1&&month > 2) {  //If it is a leap year and the month is greater than 2, total plus 1 days 
		sum++;
	}
	printf ("It is the%dth day.", sum);
}