#include<bits/stdc++.h>
using namespace std;

#define rep(i,n) for (int i = 0; i< (n); ++i)
#define vvec(m,n) vector<vector<int>> (int(m), vector<int>(n))
#define ALL(a) (a).begin(), (a).end()
#define PI 3.14159265359

using ll = long long;

int main(){
    int r;
    cin >> r;
    cout <<  r * 2 * PI << endl;
}