It is easy to sum two numbers in Brainfuck, for example by code like this:
;>; input two numbers into cells 0 and 1
[ while the number in the cell 1 is not zero
- decrement cell 1
< shift to cell 0
+ increment cell 0
> shift back to cell 1
]
<: shift to cell 0 and print the result
This task is only a bit more tricky. You need not only to get the sum of two values, but also keep these values. More exactly:
Input data has two values A and B in the single line.
Output should have the sum A+B printed into it.
Additionally after the stop the program should have values A, B, A+B in the cells 0, 1 and 2 respectively.
This problem is not that important by itself, but it shows quite important trick which may be used in building up more complicated programs in future.