#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
#define rep(i, n, s) for (int i = s; i < (int)(n); i++)

ll calcNumOfCombination(ll n, ll r) {
    ll num = 1;
    for (ll i = 1; i <= r; i++) {
        num = num * (n - i + 1) / i;
    }
    return num;
}

int gcd(int x, int y) { return (x % y) ? gcd(y, x % y) : y; }

int findSumDigits(int s) {
    int num = 0;
    int temp = 0;
    while (s) {
        temp = s % 10;
        num += temp;
        s = s / 10;
    }
    return num;
}

int main() {
    double r;
    cin >> r;
    double ans = r * 2 * 3.141592;

    cout << ans << endl;

    return 0;
}