#include <bits/stdc++.h>
#define _GLIBCXX_DEBUG
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2(i, s, n) for (int i = s; i < (int)(n); i++)
typedef long long ll;
typedef vector<vector<ll>> vvl;
typedef vector<ll> vl;
typedef vector<vector<char>> vvc;
typedef vector<char> vc;
typedef pair<ll,ll> pll;
typedef priority_queue<string> priquest;
typedef priority_queue<string,vector<string>,greater<string>> rpriquest;
ll max(ll x,ll y){
if(x>y)return x;
else return y;
}
ll min(ll x,ll y){
  return x+y-max(x,y);
}
ll lcm(ll l,ll a){
    return a/__gcd(l,a)*l;
}


int main(){
    //int a,b,c;cin >> a >> b >> c;
    //int a,b;cin >> a >> b;
    double a;cin >> a;
    //string s;cin >> s;
    cout << (a+0.0)*3.1415 << endl;
}