rust, 257 bytes

Result

AC

Duration

1846ms

Code [DL]

use std::io::*;
fn main(){let mut b=String::new();let mut c=String::new();stdin().read_line(&mut b);while !c.contains('K'){c=String::new();stdin().read_line(&mut c);print!("{}",b);}let t=c.trim_end();print!("{}{}",t,"=".repeat(b.trim_end().len()-t.len()));}

stdin

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           K                                      

stdout

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           K=======

stderr

warning: unused `std::result::Result` that must be used
 --> /tmp/code.rs:2:59
  |
2 | fn main(){let mut b=String::new();let mut c=String::new();stdin().read_line(&mut b);while !c.contains('K'){c=String::new();stdin().read_line(&mut c);print!("{}",b);}let t=c.trim_end();print!("{}{}",t,"=".repeat(b.trim_end().len()-t.len()));}
  |                                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(unused_must_use)] on by default
  = note: this `Result` may be an `Err` variant, which should be handled

warning: unused `std::result::Result` that must be used
 --> /tmp/code.rs:2:124
  |
2 | fn main(){let mut b=String::new();let mut c=String::new();stdin().read_line(&mut b);while !c.contains('K'){c=String::new();stdin().read_line(&mut c);print!("{}",b);}let t=c.trim_end();print!("{}{}",t,"=".repeat(b.trim_end().len()-t.len()));}
  |                                                                                                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this `Result` may be an `Err` variant, which should be handled

strace