Each of a1,a2,a3..aN are either 1 or -1.

The expression S = a1*a2*a3*a4+a2*a3*a4*a5+....+aN*a1*a2*a3 = 0

(* means multiplication and a1*a2 means value of a1 multiplied by value of a2)

Prove that 4 divides N.

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