#include <iostream>
#include <string.h>
#include <stdio.h>
#include <algorithm>
#include <cmath>
using namespace std;
const double PI = acos(-1.0);
const double eps = 1e-8;
inline double getlen(int n,double r)
{
    return 2.0 * r * tan( PI / n );
}
int main()
{
    ios::sync_with_stdio(0);

    #ifdef LOCAL
        freopen("in.txt","r",stdin);
    #endif // LOCAL

    double R,r,H,F,S;
    while( cin >> R >> r >> H >> F >> S ){
        double h = H / F , ans = 0 , add = ( R - r ) / F , ri  ;

        for( int i = 0 ; i < F ; i++ )
        {
            ri = add * i  + r ;
            int ll = 3 ,rr = 100000 ;
            double area ;
            int num;
            while( ll <= rr )
            {
                int mm = ( ll + rr ) >> 1;
                double tmp = getlen( mm , ri ) * h;
                if( S + eps < tmp ){
                    area = tmp , num = mm;
                    ll = mm + 1;
                }
                else{
                    rr = mm - 1;
            　　}
　　　　　　　 }
            ans += num * area;
        }

        printf("%.3lf\n",ans);
    }
    return 0;
}