A slip of paper is given to a person $A$ who marks it either with a plus sign or a minus sign. The probability of his writing a plus sign is $1/3$. A passes the slip to $B$, who may either leave it alone or change the sign before passing it to $C$. Next $C$ passes the slip to $D$ after perhaps changing the sign. Finally $D$ passes it to a refere after perhaps changing the sign. $B,C,Deachchan\ge thesignwithprobability$2//3\displaystyle.

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