zoj3058

#include<iostream>
#include<stdio.h>
#include<string.h>
#include<cmath>
#include<algorithm>
#define PI acos(-1)
#define eps 1e-6
using namespace std;
struct circle
{
    double x,y,r;
    circle(){};
    circle(double x,double y,double r):x(x),y(y),r(r)
    {}
};
double intersect(circle a,circle b)
{
    double l=sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
    if(l>=a.r+b.r||a.r==0||b.r==0)return 0;
    if(l<=fabs(a.r-b.r))return(min(a.r,b.r)*min(a.r,b.r)*PI);
    double hu=(a.r*a.r+l*l-b.r*b.r)/(2*a.r*l);
    double hu2=(b.r*b.r+l*l-a.r*a.r)/(2*b.r*l);
    double p=(a.r+b.r+l)/2;
    double are2=sqrt(p*(p-a.r)*(p-b.r)*(p-l));
    double ac1=acos(hu);
    double ac2=acos(hu2);
    double are=a.r*a.r*ac1+b.r*b.r*ac2;
    return are-are2*2;

}
int main()
{
    double x,y,r1,r2;
    circle a;
    circle b;
    circle c;
    double ans;
    while(~scanf("%lf%lf%lf",&x,&y,&r1))
    {
        a=circle(x,y,r1);
        scanf("%lf%lf%lf%lf",&x,&y,&r1,&r2);
        b=circle(x,y,r1);
        c=circle(x,y,r2);
        ans=intersect(a,c)-intersect(a,b);
        printf("%.3lf\n",fabs(ans));
    }
}