#include<bits/stdc++.h>

#define rep(i,n) for(int i = 0;i < n;++i)
#define rep2(i,n,m) for(int i = n;i < m;++i)
#define all(v) v.begin(),v.end()

using namespace std;
using ll = long long;


int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n,l;
    const double PI=3.14159265358979323846;
    cin >> n;
    l = 2*n*PI;
    cout << fixed << setprecision(8);
    cout << l << endl;


}