Tell me the area（计算几何--求两圆相交面积）

Problem Link：http://acm.hdu.edu.cn/showproblem.php?pid=1798

Tell me the area

Time Limit: 3000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 2828    Accepted Submission(s): 903

Problem Description

    There are two circles in the plane (shown in the below picture), there is a common area between the two circles. The problem is easy that you just tell