#include <cstdio>
#include <cmath>
const double pi=acos(-1);
int main()
{
    double R,r,H,S;
    int F;
    while(scanf("%lf%lf%lf%d%lf",&R,&r,&H,&F,&S)!=EOF)
    {
        double h=H/F,dr=(R-r)/F,ans=0;
        for(int i=0; i<F; i++,r+=dr)
        {
            int n=pi/atan(S/2/h/r);
            ans+=2*r*tan(pi/n)*n*h;
        }
        printf("%.3lf\n",ans);
    }
    return 0;
}