I wish

int is_even(int n)
{
    int result = -1;
    char number[8]; //should be enough
    sprintf(number, "%d", n);

    // check the number
    // TODO: handle negative numbers
    for (char *p=number; *p; p++)
    {
        if (*p=='0' || *p=='2' || *p=='4' || *p=='6' || *p=='8')
            result = 1;
        else if (*p=='1' || *p=='3' || *p=='5' || *p=='7' || *p=='9')
            result = 0;
        else {
           fprintf(stderr, "Your number is wrong!\n");
           exit(1); 
        }
    }
    return result;
}