#include <bits/stdc++.h>

#define LL long long int 
#define ii pair<int, int>
#define x first
#define y second
#define vi vector<int>
#define vvi vector<vi>
#define vii vector<ii>
#define mp make_pair
#define pb push_back
#define INF int(1e9)
#define MOD 1000000007
#define TAM int(2e5)

using namespace std;

int main () {
    ios_base::sync_with_stdio(false);

    cout << setprecision(9);

    int r;

    cin >> r;

    cout << (double)2*M_PI*r << endl;

    return 0;
}