Casimir string s consist capital latin letter a b c each turn choose follow action erase exactly letter a exactly letter b arbitrary place string letter do not adjacent erase exactly letter B exactly letter C arbitrary place string letter do not adjacent therefore turn length string decrease exactly 2 all turn independent turn Casimir choose possible actionsfor example sabcabc obtain string sacbc turn erase occurrence b second occurrence A there option turn aside particular examplefor give string s determine sequence action lead string in word Casimirs goal erase letter string Is way