int getlength(int length)
{
    if (length == 1)return 10;
    return 9 * pow(10, length - 1)*length;
}
int digitAtIndex(int index)
{
    if (index < 0)return -1;
    if (index < 10)return index;
    int curIndex = 10, length = 2, boundnum = 10;
    while (curIndex+getlength(length) < index)
    {
        curIndex += getlength(length);
        length++;
        boundnum *= 10;
    }
    int curnum = boundnum + (index - curIndex) / length;
    return to_string(curnum)[(index - curIndex) % length] - '0';
}
int main()
{
    printf("%d", digitAtIndex(13));
    return 0;
}