{a(n)} is such a sequence, satisfying,
For all a(i) ∈ {a(n)}, a(i) =1 or -1
Let S(j) = Sum[a(i) , {i ,1, j}], then for all 1<=j<=n ,S(j)>=0.
For a given n , how many {a(n)} are there?
2 of 2
added 2 characters in body 
A problem of a sequence and its sum
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