Alright, let's multiply 123456789123456789 by 987654321987654321 exactly.

To do this, we can use the standard long multiplication algorithm.

123456789123456789
x 987654321987654321
-----------------------
123456789123456789
987654321987654321
987654321987654321
987654321987654321
987654321987654321
987654321987654321
----------------------
121932631112635269
621932631112635269
758032631112635269
894132631112635269
1030232631112635269
1166332631112635269
1302432631112635269
---------------------
121932631112635269,
210765043069135559,
319197654681790879,
427630266294446199,
536062877907101519,
644495489519756839,
752928101132412159

The final result is:
121,932,631,112,635,269,210,765,043,069,135,559
