I'm having trouble to solve the following problem:
If a binary $(n,M,d)$ code with $d$ even exists, then exists an $(n,M,d)$ code with even weight for all it's codewords. (An $(n,M,d)$ code is a code consisting of $M$ words of word length $n$. The code further uses the Hamming distance $d$.)
The problem would be rather easy if this would be about linear codes, but I don't know how to solve this problem for a "general" code.
I'd be thankful if you could give me any hints how to solve this.