运行错误，无论你输入啥都只输出一种情况，求解

#include<stdio.h>
int main()
{
    int iMonth=0,iDay=0;
    printf("enter the month you want to know the days\n");
    scanf("%d",&iMonth);
    switch(iMonth)
    {
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
            iDay=31;
            break;
        case 4:
        case 6:
        case 9:
        case 11:
            iDay=30;
            break;
        case 2:
            iDay=28;
            break;
        default:
            iDay=-1;
            break;
    }
    if(iDay=-1)
    {
        printf("there is a error with you enter\n");
    }
    else
    {
        printf("2010.%d has %d days\n",iMonth,iDay);
    }
    return 0;
}