It is shown that complexity of implementation of prefix sums of m variables (i.e. functions x_1 \cdot \ldots\cdot x_i, 1\le i \le m) by circuits of depth \lceil \log_2 m \rceil in the case m=2^n is exactly
3.5\cdot2^n - (8.5+3.5(n \bmod 2))2^{\lfloor n/2\rfloor} + n + 5.