#include <bits/stdc++.h>
using namespace std;
#define _GLIBCXX_DEBUG
#define rep(i,n) for(int i=0; i<(n); i++)
#define all(n) begin(n),end(n)
using ll = long long;
using P = pair<int,int>;
const double PI = 3.14159265359;

int main() {
    double r;
    cin >> r;
    double ans = 2*r*PI;
    cout << ans << endl;
    return 0;
}