Rashed and his Kth Letter

There is always a feud between $KSK$ $Vamsi$ and $Rashed$ about who’s intelligent. In order to test $KSK$, $Rashed$ gives him a question. At first he provides $KSK$ with a string let us say $A$ – “a1a2a3……..an“. Now he asked $KSK$ to find the kth character of the string $B$ where $B$ is formed by concatenating all sub-strings with prefixes as a1,a2,a3…….,an in given sequential order as in string $