一段C程序，编译不能通过(error C2106: '=' : left operand must be l-value),求解！！

#include"stdio.h"
void main()
{   
    int year,month,day,days;
    int flag=1;
    do
    {   
        printf("请输入一个日期，格式为2008-08-08：\n");
        scanf("%d-%d-%d",&year,&month,&day);
        if((year<=0||month<=0||month>=12||day<=0))
            flag=0;
        else
        {   
            switch(month)
            {
            case 1:days=31;break;
            case 2:((year%4==0&&year%100!=0)||(year%400==0))==1?days=29:days=28;break;
            case 3:days=31;break;
            case 4:days=30;break;
            case 5:days=31;break;
            case 6:days=30;break;
            case 7:days=31;break;
            case 8:days=31;break;
            case 9:days=30;break;
            case 10:days=31;break;
            case 11:days=30;break;
            case 12:days=31;break;
            }
            if(day>days)
                flag=0;
            else
                flag=1;
        }
        if(flag==0) printf("日期输入格式有误！\n");
    }
        while(flag==0);
        printf("%d-%d-%d是一个有效的日期！\n",year,month,day);
}