A substring of a string is a successive part of the string. Given a string your task is to find out the K-th alphabet order substring of the original string.
For example, the string "ABC" contains six substrings:
"A" "AB" "ABC" "B" "BC" "C" (in alphabet order)
and string "BBC" also contains six substrings:
"B" "B" "BB" "BBC" "BC" "C" (in alphabet order)
For each test cases, output the K-th alphabet order substring.
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