#include <bits/stdc++.h>
using namespace::std;
#define all(x) (x).begin(),(x).end()
#define endl '\n'
#define int long long
#define TT template <class T>

TT istream& operator>>(istream &I, pair<T,T>& p) {return I >> p.first >> p.second;}
TT ostream& operator<<(ostream &O, pair<T,T>& p) {return O << p.first << ' ' << p.second;}
TT istream& operator>>(istream &I, vector<T>& v) {for (T &e: v) I >> e; return I;}
TT ostream& operator<<(ostream &O, const vector<T>& v) {for (const T &e: v) O << e << ' '; return O;}



void solve() {
        double r; cin >> r;
        cout << M_PI * r * 2;
}


signed main() { //[J][O][L][S][K][E][Y]
        ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
//        freopen("input.txt", "r", stdin);
        int t = 1;
//        cin >> t;
        while (t--) solve();
        return 0;
}

/*

*/