CF 600D(Area of Two Circles' Intersection-2圆面积并)

#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
using namespace std;
#define For(i,n) for(int i=1;i<=n;i++)
#define Fork(i,k,n) for(int i=k;i<=n;i++)
#define Rep(i,n) for(int i=0;i<n;i++)
#define ForD(i,n) for(int i=n;i;i--)
#define RepD(i,n) for(int i=n;i>=0;i--)
#define Forp(x) for(int p=pre[x];p;p=next[p])
#define Forpiter(x) for(int &p=iter[x];p;p=next[p])  
#define Lson (x<<1)
#define Rson ((x<<1)+1)
#define MEM(a) memset(a,0,sizeof(a));
#define MEMI(a) memset(a,127,sizeof(a));
#define MEMi(a) memset(a,128,sizeof(a));
#define INF (2139062143)
#define F (100000007)
#define pi (3.14159265358979323846264338324990142)
typedef long double ld;
typedef long long ll;
ll mul(ll a,ll b){return (a*b)%F;}
ll add(ll a,ll b){return (a+b)%F;}
ll sub(ll a,ll b){return (a-b+llabs(a-b)/F*F+F)%F;}
void upd(ll &a,ll b){a=(a%F+b%F)%F;}
int main()
{
//  freopen("D.in","r",stdin);
 // freopen(".out","w",stdout);

    cout<<setprecision(10)<<setiosflags(ios::fixed);    


    ld x1,y1,r1,x2,y2,r2;
    cin>>x1>>y1>>r1>>x2>>y2>>r2;
    ld dis=sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));

    if (dis>=r1+r2) cout<<"0\n";
    else if (dis<=abs(r1-r2)) 
        cout<<pi*min(r1,r2)*min(r1,r2)<<endl;
    else {
        ld _x=(dis*dis+r1*r1-r2*r2)/(2*dis);
        ld _x2=(dis*dis-r1*r1+r2*r2)/(2*dis);

        ld _xita=acos(_x/r1); 
        ld _xita2=acos(_x2/r2); 

        cout<<r1*r1*(_xita-sin(_xita)*cos(_xita))+r2*r2*(_xita2-sin(_xita2)*cos(_xita2))<<endl; 

    } 


    return 0;
}