switch case 语句，想知道哪里有语法错误

Question

#include<stdio.h>
         int main()
         {
             int Y, M;
             scanf("%d%d", &Y, &M);
             if (Y > 0 && M >= 1 && M <= 12)//合法输入
             {
                 if (Y % 4 == 0 && Y % 100 != 0 || Y % 400 == 0)
                     //闰年
                 {
                     switch (M)
                     {
                     case 1:3:5:7:8:10:12:
                         printf("%d", 31);
                     case 2:
                         printf("%d", 29);
                     case 4:6:9:11:
                         printf("%d", 30);
                     }
                 }

                 else if
                 {
                     switch (M)
                     {
                     case 1:3:5:7:8:10:12:
                         printf("%d", 31);
                     case 2:
                         printf("%d", 28);
                     case 4:6:9:11:
                         printf("%d", 30);
                     }
                 }
             }
             else if
             {
                 printf("error");
             }

         }

Answer 1

case 1:3:5:7:8:10:12: 这样是不可以的

                     switch (M)
                     {
                     case 3:
                     case 5:
                     case 7:
                     case 8:
                     case 10:
                     case 12:
                         printf("%d", 31);
                         break;
                     case 2:
                         printf("%d", 29);
                         break;
                     default:
                         printf("%d", 30);
                         break;
                     }