为什么结果是随机数，给位大神求教

#include <stdio.h>
int main()
{
    int moth,day,year,n,s=0,i;
    printf("请输出年月日：");
    scanf("%d%d%d",&year,&n,&i);
    moth=n;
      do
      {
      switch(moth)
      {
           case 1:
           case 3:
           case 5:
           case 7:
           case 8:
           case 10:
           case 12:day=31; break;
           case 4:
           case 6:
           case 9:
           case 11:day=30;break;
               
      }
      s+=day;
          moth--;
      }while(moth>0);
      s=s-31;//switch上面没有2，系统默认给了day=31的列式。
     if((year%4==0&&year%100!=0)||(year%400==0))//判断年是闰年
          s=s+29;
     else
          s=s+28;
      day=s+i;

          printf("%d\n",day);
          return 0;
}