The equality holds when the sum of i-th digits of numbers A and B (A[i]+B[i]<=9)
Using this write a simple program. It will be a TL program but...
try running it for k=1,2,3,4,5,6,7 and you should notice a rule how the answer for k+1 is formed.

And one more thing. You should use long arithmetics. For k=50 answer has 87 digits.

Good luck.