#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

#define FOR(i,a,b) for(int i=a;i<=b;i++)
#define FORd(i,a,b) for(int i=a;i>=b;i--)
#define fi first
#define se second
#define II pair<int,int>

const int N=1e5+10;
const ll maxN=1e18+10;
const double pi=3.14159265359;

int main()
{
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    double n;
    cin >> n;
    cout << n*2*pi;
}
    