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Heron

Time limit 1 second
Memory limit 128 MiB

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 data

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 data

Print the area of a quadrilateral with 4 decimal digits.

Examples

Input example #1
3 4 4 2 5
Output example #1
9.7997
Source SFE-2010 Variant 26