#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using ull = unsigned long long;

void solve() {
    float r;
    cin >> r;
    float pi = 3.14159;
    float ans = (float)2 * pi * r;

    cout << fixed << setprecision(4) << ans << "\n";
}

int main() {
    // #ifndef ONLINE_JUDGE
    //     freopen("input.txt", "r", stdin);
    //     freopen("output.txt", "w", stdout);
    // #endif

    // std::ios_base::sync_with_stdio(false);
    // cin.tie(NULL);
    // cout.tie(NULL);
    // int t;
    // cin>>t;

    // while(t--)
    solve();

    return 0;
}