#include <stdlib.h>
#include <bits/stdc++.h>
#include <math.h>
using namespace std;
using ll=long long;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
inline constexpr ll gcd(ll a,ll b){if(!a||!b)return 0;while(b){ll c=b;b=a%b;a=c;}return a;}
inline constexpr ll lcm(ll a,ll b){if(!a||!b)return 0;return a*b/gcd(a,b);}
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
const long long INF=1LL << 60;
long long a,b;
long long dp[100200][110];
long long w[110],v[110];


int main(){
  double a;
  cin>>a;
  cout<<2*a*M_PI<<endl;
}

