#include <bits/stdc++.h>

#define pb push_back
#define eb emplace_back

typedef long long ll;
typedef unsigned long long ull;

using namespace std;

ll bp(ll a, ll d, ll m) {
    if (d == 0) {
        return 1;
    }
    auto h = bp(a, d / 2, m);
    auto hh = (h * h) % m;
    if (d % 2) {
        hh = (hh * a) % m;
    }
    return hh;
}


void oneCase() {
    double r;
    cin >> r;
    cout << setprecision(10) <<2 * r * M_PI;
}

int main() {
//    std::ios::sync_with_stdio(false);
//    std::cin.tie(nullptr);

    int t = 1;
//    cin >> t;
    while (t--) {
       oneCase();
    }

    return 0;
}
