
#include <algorithm>
#include <vector>
#include <string>
#include <ctype.h>
#include <set>
#include <map>
#include<cstdio>
#include<queue>
#include<cstdlib>
#include<time.h>
#include<string>
#include<stack>
#include<cmath>
#include<iostream>
#include<cstring>
#include<complex>
#include<tr1/unordered_set>
#include<tr1/unordered_map>


#define FOR(i,j,k) for(int i=j;i<k;i++)
#define FORD(i,j,k) for(int i=j;i>=k;i--)
#define ll long long
//Make sure no overflow problems
//#define int long long
#define pii  pair<int, int>
#define vi vector<int >
#define pb push_back
#define mp make_pair
#define x first
#define y second
#define VAR(i,n) __typeof(n) i = (n)
#define FOREACH(i,c) for(VAR(i,(c).begin());i!=(c).end();i++)
#define FORDEACH(i,c) for( VAR(i,(c).rbegin()),i!=(c).rend();i++)
#define REP(i,n) FOR(i,0,n)
#define ld long double
const int INF = 1000000009;
const long long INFLL = (ll)INF * (ll)INF;
const ld EPS = 10e-9;
const double pi=6.283185307179586232;

using namespace std;

#pragma GCC optimize ("Ofast")
static auto _ = [] { std::ios::sync_with_stdio(0); std::cin.tie(0); std::cout.tie(0); std::cerr.tie(0); return 0; } ();

int main() {
    double i;
    cin>>i;
    cout<<i*pi;
    return 0;
}

