Sequences and Series

Let a1,a2,a3,... be a sequence of positive integers in arithmetic progression with common difference2. Also, let b1,b2,b3,.... be a sequence of positive integers in geometric progression with common ratio 2. If a1=b1=c, then the number of all possible values of c, for which the equality
2 (a1+a2+...+an) =b1+b2+....+bn
holds for some positive integer n, is ______