digits =: 24
next =: {{ (<.y)*1+y-<.y }}
catseq =: {{ (([: ".'x',~]) % 10x^<:@#) (digits+1){.;":each<.y }}
digitsOf =: {{ <. y * 10^x:x }}
solve =: {{
	lowerBound =. x:2.0
	upperBound =. x:2.9
	whilst. (digits digitsOf res) ~: (digits digitsOf start) do.
		start =. -: lowerBound + upperBound
		res =. catseq <. next^:(i.digits+1) start
		if. res < start do.
			upperBound =. start
		elseif. res > start do.
			lowerBound =. start
		end.
	end.
	start
}}

problem751 =: ({.,'.',}.)": digits digitsOf solve ''