swich-case的一个求月份天数的问题，只能打印出来31

#include<stdio.h>

int main()
{
int a,b;
scanf("%d,%d",&a,&b);
switch(b){
case 1:
printf("31");break;
case 3:
printf("31");break;
case 5:
printf("31");break;
case 7:
printf("31");break;
case 8:
printf("31");break;
case 12:
printf("31");break;
case 10:
printf("31");break;
case 4:
printf("30");break;
case 6:
printf("30");break;
case 9:
printf("30");break;
case 11:
printf("30");break;
case 2: if(a%4==0||a%100==0&&a%100!=0) printf("29");else printf("28");
}
return 0;
}