#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using ld=double;
using vll=vector<ll>;
#define FOR(i,a,b) for(ll i=a;i<(ll)b;++i)
#define ROF(i,a,b) for(ll i=a;i>=(ll)b;--i)
#define PB push_back

#define ZERO 1e-9

struct PT{
  ld x,y;
}pt[20];

PT vec(PT a,PT b){return {a.x-b.x,a.y-b.y};}

struct LN{PT s,e;}ln[4];

struct CC{PT c;ll r;}C;

int P;

bool cn[20];

void fd(LN l,int f){
  PT T;
  ld t;
  switch(f){
    case 0:
      if(fabs(C.c.y-l.s.y)>C.r)return;
      t=sqrt(fabs(C.r*C.r-pow(C.c.y-l.s.y,2)));
      T={C.c.x-t,l.s.y};
      if(T.x>l.s.x&&T.x<l.e.x)pt[P++]=T;
      T.x=C.c.x+t;
      if(T.x>l.s.x&&T.x<l.e.x)pt[P++]=T;
      break;
    case 1:
      if(fabs(C.c.x-l.s.x)>C.r)return;
      t=sqrt(fabs(C.r*C.r-pow(C.c.x-l.s.x,2)));
      T={l.s.x,C.c.y+t};
      if(T.y<l.s.y&&T.y>l.e.y)pt[P++]=T;
      T.y=C.c.y-t;
      if(T.y<l.s.y&&T.y>l.e.y)pt[P++]=T;
      break;
    case 2:
      if(fabs(C.c.y-l.s.y)>C.r)return;
      t=sqrt(fabs(C.r*C.r-pow(C.c.y-l.s.y,2)));
      T={C.c.x+t,l.s.y};
      if(T.x<l.s.x&&T.x>l.e.x)pt[P++]=T;
      T.x=C.c.x-t;
      if(T.x<l.s.x&&T.x>l.e.x)pt[P++]=T;
      break;
    case 3:
      if(fabs(C.c.x-l.s.x)>C.r)return;
      t=sqrt(fabs(C.r*C.r-pow(C.c.x-l.s.x,2)));
      T={l.s.x,0};
      if(l.s.y<l.e.y)T.y=C.c.y-t;
      if(T.y>l.s.y&&T.y<l.e.y)pt[P++]=T;
      T.y=C.c.y+t;
      if(T.y>l.s.y&&T.y<l.e.y)pt[P++]=T;
  }
}
void pre(ld lx,ld ly,ld rx, ld ry){
  ln[P=0]={{lx,ry},{rx,ry}},ln[1]={{rx,ry},{rx,ly}};
  ln[2]={{rx,ly},{lx,ly}},ln[3]={{lx,ly},{lx,ry}};
  FOR(i,0,4)pt[P++]=ln[i].s,fd(ln[i],i);
}

#define dst(a,b) sqrt(pow(a.x-b.x,2)+pow(a.y-b.y,2))
#define dst2(a,b) pow(a.x-b.x,2)+pow(a.y-b.y,2)
void tst(){FOR(i,0,P)cn[i]=dst(pt[i],C.c)>C.r+ZERO;}

ld Xp(PT a,PT b){return a.x*b.y-a.y*b.x;}

ld Dp(PT a,PT b){return a.x*b.x+a.y*b.y;}

ld tri(PT a,PT b){return Xp(vec(a,C.c),vec(b,C.c))/2;}

ld sec(PT a,PT b){
  ld da=dst2(a,C.c),db=dst2(b,C.c),cj=Xp(vec(a,C.c),vec(b,C.c)),
     dj=Dp(vec(a,C.c),vec(b,C.c)),D=cj/sqrt(da*db),A=asin(D);
  if(dj<-ZERO)A=A>0?M_PI-A:-M_PI-A;
  return A*C.r*C.r/2;
}

ld rci(ld lx,ld ly,ld rx,ld ry,CC& c){
  C=c;
  pre(lx,ly,rx,ry);
  if(C.r<ZERO)return 0;
  tst();
  ld A=0;
  FOR(i,0,P)if(!cn[i]&&!cn[(i+1)%P])A+=tri(pt[i],pt[(i+1)%P]);
      else A+=sec(pt[i],pt[(i+1)%P]);
      return fabs(A);
}

int main(){
  ios::sync_with_stdio(0);cin.tie(0);
  ll x,y,r,ax,ay,bx,by;
  cin>>x>>y>>r>>ax>>ay>>bx>>by;
  CC c={{x,y},r};
  cout<<rci(ax,ay,bx,by,c)<<endl;

  return 0;
}