In the negabinary system an i-bit binary integer, N, is expressed using positional notation as:

This is the same as conventional natural 8421 binary weighted numbers, except that alternate positions have the additional weighting +1 and -1. For example, 1101 = ( - 1 X 1 X 8) + ( + 1 X 1 X 4) + ( - 1 X 0 X 2) + ( + 1 X 1 X 1) = - 8 + 4 + 1 = - 3. The following 4-bit numbers are represented in negabinary form. Convert them into their decimal equivalents. 

Na X -1 x 20+ a X-1 X 2 +... a1 + a 1 X-1-1 X 2i-1