Math functions

Many years ago, when Paul went to school, he did not like using Heron's formula to calculate the area of a triangle, because he considered it to be very complex. One day he decided to help all the students in his school by writing a program that calculated the area of a triangle by its three sides.

However, there was a problem: as Paul did not like the formula, he did not memorize it. Help him finish this act of kindness and write the program following Heron's formula:

$$S=\sqrt{p(p - a)(p - b)(p - c)}$$ where $p=\dfrac{a + b + c}2$ is a half-perimeter of the triangle. On the input, the program has integers, and the output should be a real number corresponding to the area of the triangle.

Do NOT round the result.