calc, 112 bytes

Result

WA

Duration

1745ms

Code [DL]

=REPT(LEFT(A1;51);B2)&REPT(C3;MOD(C2-1;51))&REPT(B3;MOD(1+D2-C2;51)),=LEN(A1)/51,=FIND(B3;A1),=FIND(A3;A1)
T,K, 

stdin

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K                                                 

stdout

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KKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKK

stderr

javaldx: Could not find a Java Runtime Environment!
Warning: failed to read path from javaldx
convert /tmp/code.csv -> /tmp/out/code.csv using filter : Text - txt - csv (StarCalc)

strace