By hand it's a super easy pattern. Effectively you're taking 111,111,111 then multiplying it by each position of 1.
111,111,111
x 111,111,111
111,111,111
+) 111,111,111,0
+) 111,111,111,00
+) 111,111,111,000
Etc, repeating 9 times. Add it all together and you receive the answer:
12345678987654321
;)
Nice!
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