#include <bits/stdc++.h>
#define quick ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define F first
#define S second
#define pb push_back
#define mp make_pair
#define ll long long
#define ull unsigned ll
#define pii pair <int,int>
#define pll pair <ll,ll>
#define INF 1000000000
#define mod 1000000007
#define size(a) ((int)(a.size()))
using namespace std;
double aer(double n,double l)
{
    double p=n*l;
    double a=l/(2*tan(acos(-1)/n));
    return p*a/2;
}
void solv()
{
	double n,l,k;
	cin>>n>>l>>k;
	double a=aer(n,l);
	a*=(10000.0/k);
	double ans=sqrt(a/acos(-1));
	//cout<<ans<<endl;
	printf("%.11lf",ans);
}
int main()
{
	quick
	//freopen(".in", "r", stdin);
	int T=1; //   cin>>T;
	//cout<<aer(6,10)<<endl;
	while(T--)  solv();

	return 0;
}