#include <iomanip>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cassert>
#include <iostream>
#include <string>
#include <stack>
#include <queue>
#include <vector>
#include <map>
#include <set>
#include <algorithm>
#include <numeric>
#include <sstream>
#include <bitset>
#define repr(i,a,b) for(int i=a;i<b;i++)
#define rep(i,n) for(int i=0;i<n;i++)
#define invrepr(i,a,b) for(int i=b-1;i>=a;i--)
#define invrep(i,n) invrepr(i,0,n)

using namespace std;
typedef long long ll;

int main() {
    std::ios::sync_with_stdio(0); cin.tie(0);


    ll ans=0;
    ll anss =0;
    ll count=0;
    ll i,j,h;
    ll Q,N,K;
    
    long double a, b;
    long double x;

    vector<ll> aa;
    string S;

    ll flag=0;
    ll temp=0;

    cin  >> a;

    
    
    cout<<a*2*M_PI<<endl;

    return 0;
}