#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <functional>
#include <cmath>
using namespace std;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
using Graph = vector<vector<int> >;
// MOD
const int MOD = 1000000007;

long long GCD(long long a, long long b) {
    if (b == 0) return a;
    else return GCD(b, a % b);
}

int H, W;
int field[100][100];

const int dx[2] = {1, 0};
const int dy[2] = {0, 1};
const double PI=3.14159265358979323846;
bool seen[100][100];

int main() {
    double r; cin >> r;

    cout << 2.0*PI*r << endl;
}