#include <bits/stdc++.h>
//#include <boost/range/algorithm/find_if.hpp>
#define rep(i, n) for(int i = 0; i < (n); i++)
using namespace std;
using ll = long long;
using P = pair<int, int>;

const int MOD = 1000000007;
const int INF = 999999999;

int main(){
    
    double a;
    cin >> a;
    cout << 2 * a * 3.1415926535 << endl; 

return 0;}
/**
 * g++ code.cpp
 * ./a.out
 * shift + ctrl + i
**/ 