def solve(balls):
    r = 0
    n = len(balls)
    for i in range(0, n):
        # in half of the combinations (2**(n)), each ball is not picked
        # so we can compute its aggregated contribution to the final value as
				r += 2**(n-1)*balls[i] 
    ev = r/(2**n) # expected value
    return "%.1f"%ev
`