计算某年某月是某年的第几天

/* Note:My name is zxf */
#include "stdio.h"
void main()
{
  int year,month,day,sumdays,maxdays,xx,yy,zz,yueshu;
  int x;
   printf("请输入一个年份：");
 xx:  x=scanf("%d",&year); 
   if(x!=1||year<1990||year>9999)
   {
   	printf("年份输入有误，请重输：");
   	fflush(stdin);
   	goto xx;
   }
   
   x=0;
   printf("请输入一个月份：");
yy: x=scanf("%d",&month);
   if(x!=1||month<1||month>12)
   {
   	printf("输入月份有误，请重输：");
   	fflush(stdin);
   	goto yy;
   	}
   
  x=0;
  printf("请输入一个号数："); 
 zz: x=scanf("%d",&day);
    switch(month)
   	{
   	case 1: case 3:	case 5: case 7: case 8: case 10: case 12:
   	maxdays=31;break;
   	case 4: case 6:	case 9: case 11:	
   	maxdays=30;break;
   	case 2:
   	if(year%4==0&&year%100!=0||year%400==0)	
   	maxdays=29;
    else
    maxdays=28;break;
   	}
   	
   if(x!=1||day<1||day>maxdays)
   {
printf("输入号数有误，请重输：");
   	fflush(stdin);
   	goto zz   ;
   	}	
  for(yueshu=1;month-1>=yueshu;yueshu++) 
  {
    switch(yueshu)
   	{
   	case 1: case 3:	case 5: case 7: case 8: case 10: case 12:
   	maxdays=31;break;
   	case 4: case 6:	case 9: case 11:	
   	maxdays=30;break;
   	case 2:
   	if(year%4==0&&year%100!=0||year%400==0)	
   	maxdays=29;
 else
   maxdays=28;break;
   		}
  	sumdays+=maxdays;
  	
  	
  	
  	}	
   sumdays+=day;
   printf("%d年%d月%d日是第%d天",year,month,day,sumdays);
   
}