Heron

The sides $a,b,c,d$ and the diagonal $f$ of a convex quadrilateral are given. Find the area of a quadrilateral, using the Heron’s formula that calculates the area of a triangle.

Input

In one line five real numbers are given: $a,b,с,d,f(0<a,b,c,d,f\le 100)$. They are the lengths of segments as shown in the picture.

Output

Print the area of a quadrilateral with $4$ decimal digits.

Examples