三角形的面积
Background
Sam bought a pile of sticks from the store, but the lengths of the sticks varied, some long and some short. In order to effectively play the role of these sticks, Sam drew out three sticks at random and formed a triangle with the three sticks (it may not be possible to do so).
\(Sam\)从商场买来一堆木棒，但是木棒的长短不一，有的长，有的短。为了有效发挥这些木棒的作用，\(Sam\) 随意抽出3根木棒，用这三根木棒围成了一个三角形（也有可能围不成）。
Description
Now Sam wants to kNow the area of the triangle formed by the three sticks (double s = ... after calculation, the computer will automatically keep several decimal places). if the triangle cannot be formed by the three sticks, then "no" will be output.
现在\(Sam\)想知道这三根木棒围成三角形后的面积（直接\(double s=……\)再计算，计算机会自动保留几位小数），如果不能用这三根木棒围成三角形，则输出“\(No\)”。
Input
The three integers a, b and c respectively represent the lengths of the three sticks.
三个整数\(a,b,c\)，分别表示三根木棒的长度。
Output
When a triangle can be enclosed, the area of the triangle is output; when Not, no is output.
能围成三角形时输出三角形的面积，不能的时候则输出\(No\)。
Input
3 4 5
Output
6
Limitation
\(0<a<=100;\)
\(0<b<=100;\)
\(0<c<=100.\)
