node, 122 bytes

Result

AC

Duration

862ms

Code [DL]

a=require("fs").readFileSync(0);for(i=51;i<a.length;)a[i]|=a[i++-51];for(i-=51;a[++i]!=116;)a[i]|=a[i-1];console.log(""+a)

stdin

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K                                                 

stdout

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kkkkkkkkkkkkkkkkkkt                               

stderr

strace