1. Use integer calculation for judging if the answer is -1 or 0.
2. Use integer calculation until the last step for the area (an integer-division).
3. Use 'long long' instead of 'long' or 'int'.

Also, here are some cases which might help.

0 0 -1000 0
0 0 1000 1
1000001000.000000

-1000 0 0 1000
0 -1000 1000 -999
2002004004.004004

0 400 1000 1000
-1000 -1000 355 -187
-1

0 400 1000 1000
-1000 -1000 350 -190
0

0 400 1000 1000
-1000 -1000 353 -188
16931680400.000000

-1000 -1000 1000 1000
-1000 -1000 1000 999
31984004000.000000

-1000 -1000 1000 1000
-472 -851 379 1
5800420000.000000

More tricky cases:

0 0 3 0
1 0 1 0
0

The following testcase helps me to beat WA#33.
0 0 3 0
0 1 3 1
-1

Edited by author 27.11.2012 17:54

Edited by author 27.11.2012 17:54

Edited by author 27.11.2012 17:54

This test shouldn't be applied:
0 0 3 0
1 0 1 0
0
because it is said that window lengths are positive. And that's true, I've checked.
Or maybe a misprint?

incorrect test cases

-1000 0 0 1000
0 -1000 1000 -999
2002004004.004004

i get 2002004.0040040, but accepted.

Same here, also another test is incorrect. All others are fine.

-1000 0 0 1000
0 -1000 1000 -999
2002004.00400400

0 400 1000 1000
-1000 -1000 353 -188
16931680400.00000000