#include<set>

#include<map>

#include<queue>

#include<stack>

#include<cmath>

#include<vector>

#include<string>

#include<cstdio>

#include<cstring>

#include<iomanip>

#include<stdlib.h>

#include<iostream>

#include<algorithm>

using namespace std;

#define eps 1E-8

/*注意可能会有输出-0.000*/

#define Sgn(x) (x<-eps? -1 :x<eps? 0:1)//x为两个浮点数差的比较,注意返回整型

#define Cvs(x) (x > 0.0 ? x+eps : x-eps)//浮点数转化

#define zero(x) (((x)>0?(x):-(x))<eps)//判断是否等于0

#define mul(a,b) (a<<b)

#define dir(a,b) (a>>b)

typedef long long ll;

typedef unsigned long long ull;

const int Inf=<<;

const ll INF=1ll<<;

const double Pi=acos(-1.0);

const int Mod=1e9+;

const int Max=;

double Dis(double x1,double y1,double x2,double y2)

{

    return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));

}

double Trg(double dis)

{

    double hrep=dis*3.0/;

    return sqrt(hrep*dis/*dis/*dis/);

}

int main()

{

    int x1,y1,x2,y2;

    int n;

while(~scanf("%d %d %d %d %d",&x1,&y1,&x2,&y2,&n))

{

    if(!x1&&!y1&&!x2&&!y2&&!n)

        break;

    double ang,dis;

    dis=Dis(x1,y1,x2,y2);

    ang=360.0/n*Pi/;

    if(n==)

    {

        printf("%.6f\n",Trg(dis));

    }

    else if(n&)

    {

        dis=sqrt(dis*dis/(2.0*(-cos(ang*(n/)))));

        printf("%.6f\n",0.5*dis*dis*sin(ang)*n);

    }

    else

    {

        dis/=;

        printf("%.6f\n",0.5*dis*dis*sin(ang)*n);

    }

}

return ;

}