Suppose a triangle has side lengths $a, b$ and $c$. Find its area.

The question could have ended here, but since I am a very kind problemsetter, here is some additional information:

Heron's formula gives the area of the triangle $A$, in terms of $a$, $b$ and $c$, as $$A = \sqrt{s(s - a)(s - b)(s - c)}$$ where $s$ is the semiperimeter of the triangle, i.e. the perimeter of the triangle divided by $2$.

Input

The only line of input consists of three integers $a, b$ and $c$. $(1 \le a, b, c \le 100)$

Output

Output the area of the triangle. Your answer will be accepted if the difference between your answer and the actual answer is at most $10^{-4}$.
If you are using C++, using fixed and setprecision from <iomanip> is recommended.